Finally, another relatively common (albeit rarer) pair of synonyms derived from the root idi‑ are Sp. idiosincrasia [i.ði̯o.sin.kɾa.si̯a] and Eng. idiosyncrasy [ˌɪ.ɾɪ.ə.ˈsɪŋ.kɹə.si] (the spelling idiosyncracy, with a c, is also often found but it is dispreferred since it is etymologically erroneous). Actually, the source word was a compound, since it was composed of two roots, as we shall see. Both of these words mean now ‘a mode of behavior or way of thought specific to an individual’ or, derived from that meaning, ‘a distinctive or peculiar characteristic of something’ (COED). Some synonyms of Eng. idiosyncrasy are peculiarity, mannerism, quirk, characteristic, quality, and eccentricity.
Both cognates are rare and fancy, though the Spanish word is probably less common. Therefore, Eng. idiosyncrasy is often better translated by the synonyms rareza and mania, when referring to a person’s weakness or eccentricity, and peculiaridad (cf. Eng. peculiarity) and particularidad, speaking of things.
English idiosyncrasy is first attested in the early 17th century and it was a loanword from Fr. idiosyncrasie, a learned word that French borrowed from Greek in the late 16th century. Spanish idiosincrasia did not appear in the DRAE until 1869, though it first appeared in another dictionary in 1787, a hundred years after it appeared in French and two hundred years after it appeared in French. Thus, there is little doubt that Spanish also obtained this word through French, just like English did. Although English borrowed the word from French, the French version of this word is also less common than the English one. Like Spanish, Modern French prefers alternatives such as manie and particularité, cognates of the Spanish words we just saw. The term idiosyncrasy has been used to some extent as a technical term in different academic disciplines, such as 19th century medicine, pharmacology, psychiatry, economics, and linguistics.
The source of these words is Ancient Greek ἰδιοσυγκρασία (idiosunkrasía) ‘one’s own (peculiar) temperament’, a word derived from ἴδιος (ídios) ‘one’s own’ plus σύγκρᾱσις (súnkrasis) ‘mixture, temperament (mixture of personal traits)’, a word that was derived from the prefix σύν (sún) ‘together’ (cf. Eng. syn‑ ~ Sp. sin‑), the noun κρᾶσις (krâsis) ‘mixture, blending, etc.’, and the suffix ‑ῐ́ᾱ (‑íā) that formed feminine abstract nouns from the stems of adjectives (or, rarely, from the stems of verbs).
The cognate adjectives for these two nouns are Sp. idiosincrásico/a and Eng. idiosyncratic [ˌɪ.ɾɪ.ə.sɪŋ.ˈkʰɹæ.ɾɪk] (cf. Fr. idiosyncrasique [i.djo.sɛ̃.kʀa.zik]). Notice that the English word has a t where Spanish (and French) has s. This is due, no doubt, to a mistaken etymology for this word, the same one that resulted in the alternate spelling idiosyncracy for the noun. It seems that this adjective was formed in English first by adding the adjectival suffix ‑ic to the noun idiosyncrasy in the mid-18th century, though the variant idiosyncratical, with the double adjectival suffix ‑ic‑al, is attested in the mid-17th century already. This is because English often derived adjectives from Latin verbs by adding the two Latinate derivational suffixes that formed adjectives, namely ‑ic‑ and ‑āl‑, resulting in many English words ending in ‑ical, such as acoustical, analytical, and anatomical. Note that the equivalent Spanish words only contain the suffix ‑ic‑, plus the gender inflection: acústico/a, analítico/a, and anatómico/a. From English, the word idiosyncratic passed on to French and Spanish, though these two languages corrected the mistaken etymology and replaced the English t with an s.
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 In phonology, an idiosyncratic property of the sound system is one that contrasts with a systematic regularity.
 In Greek grammar, the word crasis was used for ‘the contraction of two adjacent vowels into one long vowel or diphthong, esp. at the end of one word and beginning of the next’ (SOED). In phonetics, Eng. crasis has been used more genereally to mean ‘a contraction of two adjacent vowels into one long vowel or diphthong, for example the reduction of words in ancient Greek from three syllables to two’ (Oxford).
 English words that end in ‑cy come from New Latin words that end in either ‑c‑ia or ‑t‑ia, as do Spanish words that end in ‑cia and ‑cía. When the adjectival suffix ‑ic‑ is added to an English word that ends in ‑cy that comes from Lat. ‑tia, such as democracy (from Medieval Latin democratia), then the t is restored’ (cf. Eng. democratic).