a characteristic of thought or behaviour peculiar to an individual or group; an eccentricity
characteristic peculiarity of habit or structure
an individual's allergic sensitivity or reaction to a drug, food, etc
idiosyncratic /-ʹkratik/ adj
idiosyncratically /-ʹkratikli/ adv
[Greek idiosynkrasia, from idio- + synkerannynai to blend, from syn- + kerannynai to mingle, mix]