Words derived from Lat. frāter
Sp. fraterno/a and fraternal and Eng. fraternal
Spanish has a couple of learned adjectives derived from the Latin noun frater, one of which has an English cognate of the same provenance. One of them is fraterno, from Lat. frāternus ‘fraternal, brotherly’, as in Sp. amor fraterno ‘brotherly love’ (frātern‑us). Lat. frāternus seems to be derived from the noun frāter plus the adjectival ‑ān‑ suffix. In other words, it contains the morphemes frātr+an+us and is thus an irregular formation, probably influenced by the nominative wordform frāter. This learned adjective was borrowed into Spanish in the 15th century. The feminine form of this adjective in Spanish is fraterna, as in amistad fraterna ‘brotherly friendship’.
From the Latin adjective frāternus, a synonymous adjective was derived in Medieval Latin by the (redundant) addition of the adjectival ending ‑āl‑, resulting in the word frāternālis (frātern‑āl‑is), accusative frāternālem. From this word, we get Sp. fraternal [fɾa.t̪eɾ.ˈnal], a synonym of Sp. fraterno/a, and its English cognate fraternal [fɹə.ˈtʰɜɹ.nəl], which came into English in the 15th century. Both words probably came in through French fraternel (fem. fraternelle), which was already present in Old French by the 12th century.
Verbs derived from Lat. frater
There are some verbs in English and Spanish that contain the root of the Latin noun frater, such as the cognates Sp. fraternizar [fɾa.t̪eɾ.ni.ˈθaɾ[ ~ Eng. fraternize [ˈfræ.təɹ.naɪ̯z]. These verbs come from Medieval Latin frāternizāre, derived from the Latin adjective frāternus and the verbal suffix ‑iz‑, a loanword from Ancient Greek Ancient Greek ‑ίζειν (-ízein) (frātern‑iz‑ā‑re). English borrowed this verb from French quite late, in the 17th century, and so did Spanish, probably around the same time or a little bit later.
English-Spanish dictionaries typically say that fraternizar and fraternize are perfectly equivalent (i.e. “very good friends”), since both can mean something like ‘to associate with others in a brotherly or congenial way’ (AHD). Actually, English fraternize has a second sense that the Spanish cognate does not have, one that seems to derive from its use in the military. This sense is ‘to associate on friendly terms with an enemy or opposing group, often in violation of discipline or orders’ (AHD), and in particular, ‘to have relations with foreign women when stationed overseas’. Spanish fraternizar does not have that sense (but see below).
Spanish has a somewhat archaic synonym of fraternizar, namely confraternar, also meaning ‘to treat each other with love and friendship like brothers’ (DUEAE). This word, which contains the prefix con‑ ‘with’, is older than fraternizar. There is also a blend in Spanish of fraternizar and confraternar, namely the verb confraternizar, which presumably was influenced by the French word confraternité, a learned 13th century derivation based on an older confrère (see below). The verb confraternizar is often used with the narrow sense we just saw Eng. fraternize has, namely ‘to associate on friendly terms with an enemy or opposing group’.
Sp. fraternidad, Eng. fraternity, and related words
The cognate nouns Eng. fraternity [fɹə.ˈtʰɜɹ.nɪ.ɾɪ] and Sp. fraternidad [fɾa.t̪eɾ.ni.ˈðað] are learned words that go back to Lat. frāternitātem (accusative of frāternitās), which meant ‘brotherhood, fraternity; the relationship of brothers’, a word derived from the adjective frāternus we just saw that meant ‘brotherly, fraternal’. English borrowed it in the 14th century through Old French fraternité, a learned word in that language, first attested in the 12th century.
Eng. fraternity of course had to compete with the native word brotherhood, which has basically the same meaning. Although brotherhood, with the ending ‑hood, is only first attested in the 15th century, it comes from an earlier form broþerhede in Middle English. In Old English, we find the equivalent forms derived from the word for brother: broþerrede, broðorscipe (i.e. brothership), and broðorsibb, all meaning something like ‘fellowship, brotherhood, kinship of brothers’.
Spanish fraternidad appears in writing for the first time in the 15th century and is also, no doubt, a loanword from French, who borrowed it from Latin first. These two nouns are not perfect friends either, despite being cognates. The two are synonyms when fraternity is used with the sense of ‘the quality or condition of being brothers; brotherliness’ (AHD). But Eng. fraternity can have other senses that Sp. fraternidad does not have. English fraternity can have a sense of ‘association’ or ‘society’, that is, ‘a group of people sharing a common profession or interests’ (COED). The Spanish equivalent of that sense would be asociación, hermandad, or cofradía. Let us look at the last two of these words, which do have the ‘brother’ component incorporated in them.
In American English, there is an added sense for the word fraternity, which is probably the most common sense of this word in the US, at least in the academic sphere. The sense refers to ‘a chiefly social organization of men students at a college or university, usually designated by Greek letters’ (AHD). The feminine equivalent is sorority, from the Latin word for sister, soror (see next section). Fraternities and sororities do not exist in the academic Spanish-speaking world, so there are no words to describe them. One dictionary offers the option club de estudiantes (AES). Spanish speakers in the US often use the cognate fraternidad to express this meaning, which is an estadounidismo, that is, a Spanish word (or use of a word) that is found only in United States Spanish, (cf. Part I, §2.8).
One native Spanish way to express a meaning similar to that of (borrowed) fraternidad is the noun hermandad, first attested in the 12th century. This noun is made up of the stem of the noun hermano (herman‑) and the suffix ‑dad (herman+dad). Thus, it would seem that this word is a calque, a word created in Spanish by analogy with the Latin word frāternĭtātem, since the Spanish suffix ‑idad is a patrimonial descendant of Lat. ‑ĭtātem.
In the Middle Ages, the word hermandad was used to refer to certain ‘voluntary organizations formed in Spain during the 13th, 14th, and 15th centuries to maintain public order and resist the depredations of the nobles and later to exercise general police functions’ (WU). Modern hermandad still has that meaning of ‘brotherly association’, as well as the same abstract sense of ‘close relationship’ or ‘a feeling of kinship and closeness’ that Eng. brotherhood has. Because the root herman‑ is not intrinsically masculine, the word hermandad can also mean ‘sisterhood’ in addition to ‘brotherhood’, at least in theory.
The Spanish noun cofradía is also quite old. It means ‘brotherhood, association, guild’. It is derived from the noun cofrade, which now means primarily ‘member of a guild, brotherhood, etc.’. Its literal meaning is ‘co-brother’ or ‘fellow brother’. This word is first attested as confrare (12th century), confradre (13th c.), confrade (14th c.) and, finally cofrade (16th c.). In the earlier forms, we can see clearly that this word is formed from the prefix con‑ ‘with’ and a patrimonial derivation of Lat. frater ‘brother’ or, rather, the accusative wordform of this Latin noun: frātrem. (Remember that it was common for Latin words that had two r’s in them to lose one by the time the word got to Old Spanish, cf. Part I, Chapter 10.) The verb confraternar, mentioned earlier as a synonym of fraternizar, is related to this noun, though it seems to be more learned.
By the way, in addition to the learned noun fraternité (a 12th century loan from Lat. fraternitās, as we have seen), French also has the word confrérie ‘brotherhood’, a cognate of Sp. cofradía. This word still exists in French but it now used only in religious and freemasonry contexts. An earlier version was confrarie (c. 1190), which some see as coming from Medieval or Church Latin confratria, again, derived from the prefix con‑ ‘with’ and the noun fratria ‘siblings, brothers and sisters’ (cf. Latin phrātria, from Greek φρατρία (phratría) ‘tribe, clan’, derived from φράτηρ (phrátēr) + ‑ία (-ía).) This word also originally had the same two meanings that brotherhood and fraternity have, the abstract one (‘brotherliness’) and the concrete one (‘association, society’).
Other words from the Latin root frātr‑
There is another way in which the Latin word frāter has made it into English and Spanish, namely Spanish fraile and its English cognate friar, both used to refer to a Catholic monk, a member of a Roman Catholic religious order of men, in particular one of the four mendicant orders: Augustinians, Carmelites, Dominicans, and Franciscans (Sp. agustinos, carmelitas, dominicos, and franciscanos). The Spanish loanword fraile substituted a patrimonial word fradre in Old Spanish (attested in the 11th century), that descended from Lat. frātrem, the accusative wordform of frāter.
English borrowed the word friar in the 13th century from O.Fr. frere, a word that meant both ‘brother’ and, in a religious context, ‘friar’. This French word was a patrimonial descendant from Lat. frater ‘brother’. Modern French has retained the patrimonial word frère [ˈfʀɛʀ] to this day, which still has both meanings, ‘brother’ and, in the right contexts, ‘friar’. Note that the English word brother can also be used in both contexts, though interestingly, in the religious one, the plural of brother is often not brothers but brethren, the Old English plural form of this noun.
Spanish fraile is a medieval borrowing from Occitan fraire, which is how Latin frater had changed in this Romance language of southern France. It is thus a cognate of northern and standard French frère. The word suffer dissimilation of one of the two r’s in Spanish (cf. Part I, Chapter 10, §10.4.5). The word hermano in Spanish and the word brother in English can also be used in a special sense to refer to certain members of Christian orders, particularly Catholic ones, though calling fellow Christians brothers and sisters in English and hermanos in Spanish (hermano and hermana in the singular) is something that goes back to the beginnings of Christianity.
Finally, derived from the same Latin word frāter, Spanish has the words fratricidio and fratricida, and English has the word fratricide [ˈfɹa.tɹɪ.saɪ̯d], which is equivalent to both Spanish words. Spanish fratricidio means ‘the act of killing one’s brother’. It comes from Late Latin fratrĭcīdĭum ‘the killing of a brother’ or fratricide in English. Sp. fratricida refers to the person who comits the act of killing their brother, a meaning that is also expressed with the word fratricide in English, though this use of the word is not as common as the former one. It comes from Lat. frātrĭcīda, with the same meaning, ‘one who murders a brother, a fratricide’ (L&S) (in Late Latin, frātriscīda is also attested). This word is formed from the root frātr‑ ‘brother’ and the variant ‑cīd‑ of the morpheme ‑caed‑ of the verb caedĕre ‘to kill’ (cf. frātr‑ĭ‑cīd‑a, with the linking vowel ‑ĭ‑). In addition of being a noun, Sp. fratricida can also be an adjective, which translates into Eng. as fratricidal.
These two related Spanish words seem to have entered the language in the 16th century. Eng. fratricide first entered the English language in the mid-15th through Old French with the ‘person’ sense (Lat. frātrĭcīda) but by 100 years later, it was being used with the ‘act’ sense as well (Lat. fratrĭcīdĭum). Interestingly, Old English already had words for both a brother’s killer and the act of killing a brother, namely compounds such as broðorbana and broðorslaga (killer) and broðorcwealm and broðorslege (act).
 It was much later that the French word fraternité became part of the official motto of France: Liberté, Egalité, Fraternité (Liberty, Equality, Fraternity). Although it has its origin in the French Revolution (1789-1799), it did not become the official motto until the end of the 19th century during the Third Republic. It is also the motto of Haiti, which around the same time became independent from France (1804) after fighting its own Haitian Revolution (1791-1804).
 The pattern we just saw extends to other words that come from Latin words derived with the ‑cīd‑ morpheme, though in the other cases, the English word can only refer to the person, not the act. Thus, we have Sp. homicidio ‘homicide’ and homicida ‘n. murderer; adj. homicidal’ or suicidio ‘suicide’ and suicida ‘n. suicide victim; adj. suicidal’, cf. Part I, Chapter 5, §5.11.1.