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c.1600, from French idiosyncrasie, from 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" (see rare (adj.2)). Originally in English a medical term meaning "physical constitution of an individual." Mental sense first attested 1660s.
idiosyncrasy id·i·o·syn·cra·sy (ĭd'ē-ō-sĭng'krə-sē)
A structural or behavioral trait peculiar to an individual or a group.
A physiological or temperamental peculiarity.
An unusual individual reaction to food or a drug.