idiosyncrasy

Etymology

First attested in 1604, in modern sense since 1665, from Ancient Greek ἰδιοσυγκρασία (idiosunkrasía, “one’s own temperament”), from ἴδιος (ídios, “one’s own”) + σύν (sún, “together”) + κρᾶσις (krâsis, “temperament”). Analyzable as idio- + syn- + -crasy.

noun

1. A behavior or way of thinking that is characteristic of a person.
2. A language or behaviour that is particular to an individual or group.
3. (medicine) A peculiar individual reaction to a generally innocuous substance or factor.
4. A peculiarity that serves to distinguish or identify.
   He mastered the idiosyncrasies of English spelling and speech.