idiosyncratic n.

c. 1600, from French idiosyncrasie, from Latinized form of Greek idiosynkrasia "a peculiar temperament," from idios "one's own" (see idiom) + synkrasis "temperament, mixture of personal characteristics," from syn "together" (see syn-) + krasis "mixture," from PIE root *kere- "to mix, confuse; cook" (see rare (adj.2)).

Originally in English a medical term meaning "physical constitution of an individual;" mental sense "peculiar mixture" of the elements in one person that makes up his character and personality first attested 1660s. In modern use, loosely, one's whims, habits, fads, or tastes. Sometimes confused in spelling with words in -cracy, but it is from krasis not kratos.

Middle English -ik, -ick, word-forming element making adjectives, "having to do with, having the nature of, being, made of, caused by, similar to," from French -ique and directly from Latin -icus or from cognate Greek -ikos "in the manner of; pertaining to." From PIE adjective suffix *-(i)ko, which also yielded Slavic -isku, adjectival suffix indicating origin, the source of the -sky (Russian -skii) in many surnames. In chemistry, indicating a higher valence than names in -ous (first in benzoic, 1791).

In Middle English and after often spelled -ick, -ike, -ique. Variant forms in -ick (critick, ethick) were common in early Modern English and survived in English dictionaries into early 19c. This spelling was supported by Johnson but opposed by Webster, who prevailed.