From the Latin sitting verb sĕdēre, another verb was derived by means of the prefix ob‑ that means ‘towards’ or ‘against’, depending on the context. The verb was obsĭdēre, which literally meant ‘to sit opposite to’ and whose principal parts were obsĭdĕo, obsĭdēre, obsēdi, and obsĕssus. In this case too, the allomorph of the sĕd‑ root in the first two principal parts was ‑sĭd‑, as you can see. In the two other principal parts, the vowel was either ‑ē‑ or ‑ĕ‑, just like in the base verb sĕdēre. In poetry, this verb was used with the meaning ‘to sit, stay, remain, abide anywhere’ but, most importantly, in the context of war it had the meaning ‘to hem in, beset, besiege, blockade’ (CTL).
The prefix ob‑ was also added at some point to sĕdēre’s companion verb sīdĕre, resulting in the verb obsīdĕre, found mostly in poetry, that also meant ‘to beset, besiege, blockade’ and ‘to occupy, take possession of’ (L&S) and thus could be a synonym of obsĭdēre. The only two attested principal parts of this verb are obsīdō and obsīdĕre.
Finally, we should mention that there is a related deponent verb obsĭdĭāri ‘to lie in wait for, to waylay’ (principal parts: obsĭdĭor, obsĭdĭāri). A deponent verb is one that has passive forms but active meanings (cf. Part I, Chapter 8, §184.108.40.206). This verb is not attested until fairly late, in the post-Augustan period.
From the verb obsĭdēre, Latin derived two synonymous nouns: obsĭdĭum and obsĭdĭo, both of which meant ‘siege, blockade’. It isn’t clear what the difference in meaning between the two was, if any, or whether they were used at different times or in different places. We saw the first of these words when we saw that Sp. asedio, one of the two words for ‘siege’, is said to derive from the noun obsĭdĭum (ob‑sĭd‑ĭum). Presumably asedio was a patrimonial word, given the ĭ to e sound change, but the change in the prefix’s vowel from o to a is not at all common. This Latin noun was derived from the present stem of the verb, ob‑sĭd‑ and the suffix ‑ĭ‑um, used at one time form abstract nouns.
Regarding the word obsĭdĭo, its genitive wordform was obsĭdĭōnis and thus its regular stem was ob‑sĭd‑ĭōn‑. The derivational ending of this word looks just like the suffix ‑ĭōn‑ that we have seen so often that derives third-declension abstract nouns from verbs (‑ĭo in the nominative case), but there is a major difference. In this case, this suffix attaches itself not to the passive participle (supine) stem of the verb, obsĕss‑, but rather to the present stem, in this case obsĭd‑. There are very few examples of abstract nouns derived by means of this suffix from the present stem of a verb. Two of them are lĕgĭo-lĕgĭōnis ‘legion’ and rĕgĭo-rĕgĭōnis ‘region’.
However, as we will see below, there is also a noun that is derived from the passive participle stem by means of the suffix ‑ĭōn‑, namely obsĕssĭo (gen. obsĕssĭōnis; regular stem: ob‑sĕss‑ĭōn‑). No doubt, you will recognize the cognates Eng. obsession ~ Sp. obsesión as coming from that derived noun. But before turning to Eng. obsession ~ Sp. obsesión, let us mention that English did borrow at one point, in the middle of the 15th century, the noun obsidion ‘siege’, through French, where it was a learned loanword from Latin obsĭdĭo-obsĭdĭōnis. Its use was rare even then and the word is now obsolete.
In addition to the noun obsidion, English also borrowed from French an adjective derived from the noun. The English adjective was obsidional ‘pertaining to a siege’, borrowed in the 16th century from French obsidional, itself borrowed in the 15th century from Lat. obsĭdĭōnālis, an adjective derived by means of the third declension adjectival suffix ‑āl‑ (ob‑sĭd‑ĭōn‑āl‑is). There is even a synonym of obsidional that was created in English in the late 19th century, namely obsidionary, formed from obsidion and the Latinate suffix ‑ary that we saw in the previous section.
There is another, related adjective in English, namely obsidious ‘besieging; besetting’. According to the OED, this adjective was created in the early 17th century, in English, out of the Latin noun obsĭdĭum that gave us Sp. asedio by the addition of the Latinate suffix ‑ous (from Lat. ‑ōs‑) that formed first/second declension adjectives from nouns. Few English dictionaries mention this adjective and the few that do mention that it is rare. However, for some reason, just as in the case of the other English words containing the same stem, none of dictionaries says this word is either archaic or obsolete.
Although the word obsidion and its relatives will not be familiar to most speakers of English, they will probably remind many speakers of a much more common word, namely obsidian, which refers to ‘a hard, dark, glass-like volcanic rock formed by the rapid solidification of lava without crystallization’ (COED). (Every time I type the word obsidion, Word replaces it with the word obsidian.) There is no connection between the two words, however. Eng. obsidian (Sp. obsidiana) is a 17th century loanword from Latin, but its form, what makes it look like the noun obsidion, is actually due to a a misreading that a copyist made at some point when transcribing the Latin word into Latin. The actual, original Latin word was obsĭānus, not obsĭdĭānus, an adjective derived from Obsĭus, a Roman surname and, according to Pliny the Elder, the name of the person who first discovered or described this type of stone in Ethiopia (Obsĭ‑us +‑ān‑ → obsĭ‑ān‑us).
Let us return now to the cognates Eng. obsession [əb.ˈsɛ.ʃən] ~ Sp. obsesión [ob.se.ˈsi̯on]. These words are loanwords ultimately from Lat. obsessĭo, a noun derived from the stem obsess‑ of the passive participle obsessus of the verb obsĭdēre by means of the noun-forming suffix ‑ĭōn‑ (genitive case: obsessĭōnis; regular stem: ob‑sess‑ĭōn‑). Curiously, in Latin this word was a synonym of obsĭdĭo-obsĭdĭōnis and obsĭdĭum, and thus meant ‘siege, blockade’. That was also the meaning the word obsession had when this word was borrowed into English in the early 16th century from French, which itself had borrowed it from Latin in the 15th century. However, by 1590, the French word had come to mean ‘state or condition of someone besieged by a demon’, a preliminary stage to demonic possession, but without the demon actually inhabiting the body. That meaning was transferred to the English cognate soon thereafter.
To understand the original meaning of Lat. obsessĭo, and its companion possessĭo, it helps to understand how these words were used in Roman military terminology. In Roman warfare, there were two stages of taking over an enemy city. The first one was obsession (obsĭdēre, obsessĭo), the siege, and the second one possession, or taking it over by force after the walls have been breached. In the next section we will explore the word possession and the verb possĭdēre that this noun is derived from, since these words, which are actually compounds, are also derived from the verb sĕdēre and also contains the root sĕd‑.[i]
The words obsession and possession are used in Christian terminology to refer to supposed demonic influences on people but, obviolsy, they are mere analogues of the Roman war terms we just saw. In obsession, a demon besieges a person and in possession the demon takes over the person. By the late 17th century the word obsession was being used in non-religious contexts to refer to anything that absorbs a person’s mind. In Modern English, the word obsession has lost the military and religious senses and can be defined as ‘an extreme unhealthy interest in something or worry about something, which stops you from thinking about anything else’ (DOCE) or, with more of a psychological twist, as a ‘compulsive preoccupation with a fixed idea or an unwanted feeling or emotion, often accompanied by symptoms of anxiety’ (AHD).
Sp. obsesión first appeared in the DRAE’s 1780 edition of the DRAE. It is obviously a loanword, which probably came into the language through French, just like its English cognate did, not directly from Latin. The borrowing probably took place after the meaning had already changed to its current one in the late 17th century, for there is no sign that the Spanish word obsesión ever meant anything other than what it means today, which is also exactly what it means in Modern French and Modern English.
These cognates that come from Lat. obsessĭo have a more technical use in the field of psychology and psychiatry in the modern languages, one which can be defined as follows:
An obsession is the inability of a person to stop thinking about a particular topic or feeling a certain emotion without a high amount of anxiety. When obsessed, an individual continues the obsession in order to avoid the consequent anxiety. In the case of obsessive compulsive disorder (OCD), the individual may have only the obsessions, compulsions or both. An example of an obsession in OCD is a person who can't stop thinking about dirt or germs that they could come into contact with. In this case, thinking about the dirt and/or germs is the obsession.[ii]
English synonyms of the word obsession are preoccupation, fixation, compulsion, and mania. Spanish synonyms of obsesión are manía, fijación, and obcecación. The first two are cognates of Eng. mania and fixation, respectively. In Spanish colloquial speech, perra and neura are common synonyms, at least in some dialects (Clave).
Both English and Spanish have words related to or even derived from the nouns Eng. obsession ~ Sp. obsesión. One pair of words that the two languages have in common are the cognate adjectives Eng. obsessive [əb.ˈsɛ.sɪv] ~ Sp. obsesivo/a [ob.se.ˈsi.βo]. The word *obsessīvus did not exist in Latin, though it could have existed, since it is properly formed by the addition of the first-second conjugation adjectival suffix ‑īv‑ to perfect passive participial stems of verbs. But we know that these adjectives were created in the field of psychology in the early 20th century, in one of the modern languages and then copied in the others, out of the stem obsess‑ of the word obsession and the Latinate suffix Eng. ‑ive (in French, it is masc. ‑if, fem. ‑ive; in Spanish,‑ivo/a). Its main meaning is ‘of, relating to, characteristic of, or causing an obsession’ (AHD). From this adjective, English has derived the adverb obsessively (Sp. obsesivamente) by the addition of the native (Germanic) suffix ‑ly and the noun obsessiveness, by the addition of the native suffix ‑ness (there is no Spanish equivalent for this latter word, though *obsesividad would be a likely candidate).
Let us look now at the verbs related to these nouns. English has borrowed the Latin verb obsĭdēre not once but twice. A failed attempt was the borrowing as by some author in the late 17th century of this verb as obside, with the meaning ‘to beset, invest, surround, encompass’ (OED). Needless to say, the verb is now obsolete. But before that failed attempt, English had already borrowed the same verb in the early 16th century, from the usual source, namely the verb’s passive participle, the verb form obsessus, resulting in the verb obsess. When the verb was first borrowed it had the expected meaning, namely ‘to besiege a fortress’. By the mid-16th century the verb had already acquired the sense of a demon besieging a person, much like the noun obsession had changed meanings. By the 17th century, the past participle of this verb, obsessed, had already become an adjective with the meaning ‘haunted, tormented by an evil spirit’. Then, in the 20th century, the verb acquired its modern transitive and intransitive meanings related to being constantly preoccupied with something.
Dictionaries tell us that the verb obsess has a transitive meaning, ‘to preoccupy the mind of excessively’, and an intransitive one, derived from the former, ‘to have the mind excessively preoccupied with a single emotion or topic’ (AHD). The latter is indeed a use of the verb obsess, as in as in She always obsesses about her hair, though this use is rare. But English obsess is not really used transitively anymore and what dictionaries term as the passive use of the transitive verb (be/become/get obsessed by) is really an adjectival use of obsessed, the past participle of the verb, not a true passive participle, as shown by the fact that the source of the obsession is typically coded with a with-phrase and not a by-phrase, as in She is obsessed with her hair. Note that the transitive sentence Her hair obsesses her sounds a bit odd and is of dubious grammaticality.
The equivalent of Eng. obsess in French is obséder, which was borrowed and adapted from Lat. obsĭdēre. Spanish, on the other hand, never borrowed this Latin verb. Instead, it created a verb obsesionar out of the noun obsession (obsesion‑ar). The verb was created by some unknown author sometime in the second half of the 19th century, but it did not appear in a dictionary until 1917, and in the DRAE until 1927, after which time it becomes very common in the language.
Spanish obsesionar is equivalent to transitive English obsess, meaning something like ‘to cause an obsession’. The difference is that this verb is indeed used as a transitive verb, as in Lo obsesiona el fútbol (or El fútbol lo obsesiona) ‘He’s obsessed with soccer’. As usual, this transitive verb can be used ‘pronominally’ (reflexively), as obsesionarse (por/con), which translates as to get obsessed (by/with/about), as in Se obsesiona por/con cualquier cosa ‘He becomes obsessed with just about anything’. Spanish also uses the past participle of this verb, obsesionado/a, as an adjective in constructions such as estar obsesionado/a (con), as in Está obsesionada con salir a comer ‘She’s obsessed about going out to eat’.
 Lat. lĕgĭo-lĕgĭōnis ‘legion’ is derived from the present verb lĕgĕre ‘to choose; to collect; to read’ (lĕgō, lĕgĕre, lēgī, lēctus); and (2) rĕgĭo-rĕgĭōnis ‘direction; boundary; region’, from the verb rĕgĕre ‘to keep/lead straight; to guide, direct’ (regō, regĕre, rēxī, rēctus). Other such nouns are căpĭo ‘a taking’, contāgĭo ‘a touching’, īnflectĭo ‘a bending; inflection’, rĕlĭgĭo ‘sense of right, moral obligation, duty; sense of religious obligation, religious sanction, duty to the gods; etc.’ (CTL), suspīcĭo ‘mistrust, distrust, suspicion’, and ūnĭo ‘oneness, unity, union’.
 Pliny the Elder (23-79 CE), whose Roman name was Gaius Plinius Secundus, wrote about this in his famous Naturalis Historia (Natural History), the first encyclopedia ever written. In this book he mentioned that “Among the various kinds of glass, we may also reckon Obsian glass, a substance very similar to the stone which Obsius discovered in Æthiopia. The stone is of a very dark colour, and sometimes transparent; but it is dull to the sight,…” (Chapter 67: “Obsian glass and obsian stone”).
[i] Source: Obsession : a history, by Lennard J Davis. Chicago, Ill. : University of Chicago Press, 2009.
[ii] Source: AlleyDog.com, Psychology students’ best friend https://www.alleydog.com/glossary/definition.php?term=obsession (accessed: 2018.01.06)