id·i·o·syn·cra·sy / ˌidēəˈsingkrəsē/ • n. (pl. -sies) (usu. idiosyncrasies) a mode of behavior or way of thought peculiar to an individual: one of his little idiosyncrasies was always preferring to be in the car first. ∎  a distinctive or peculiar feature or characteristic of a place or thing: the idiosyncrasies of the prison system. ∎  Med. an abnormal physical reaction by an individual to a food or drug.

idiosyncrasy XVII. — Gr. idiosugkrāsiä, -krāsis, f. IDIO- + súgkrāsis commixture, tempering, f. sún SYN- + krāsis mixture.

idiosyncrasy An abnormal reaction to a drug or other foreign substance shown by an individual, which is usually genetically determined. An individual that shows immunological idiosyncrasy is said to be hypersensitive to a particular substance, agent, etc.

idiosyncrasy (idi-oh-sink-răsi) n. an unusual and unexpected sensitivity exhibited by an individual to a particular drug or food.
—idiosyncratic (idi-oh-sin-krat-ik) adj.