The Collaborative International Dictionary of English v.0.48:

Idiosyncrasy \Id`i*o*syn"cra*sy\, n.; pl. Idiosyncrasies. [Gr.
   ?; 'i`dios proper, peculiar + ? a mixing together, fr. ? to
   mix together; ? with + ? to mix: cf. F. idiosyncrasie. See
   Idiom, and Crasis.]
   A peculiarity of physical or mental constitution or
   temperament; a characteristic belonging to, and
   distinguishing, an individual; characteristic susceptibility;
   idiocrasy; eccentricity.
   [1913 Webster]

         The individual mind . . . takes its tone from the
         idiosyncrasies of the body.              --I. Taylor.
   Idiosyncratic

WordNet (r) 3.0 (2006):

idiosyncrasy
    n 1: a behavioral attribute that is distinctive and peculiar to
         an individual [syn: idiosyncrasy, foible, mannerism]