an odd habit or behavior of an individual; an eccentricity
Her most striking idiosyncrasy is her habit of twisting a strand of her hair with her fingers whenever she is deep in thought.
idios (one's own)#synkrasis (temperament, mixture of personal characteristics) (Greek)
Idiot in sync with the crazy people are idiosyncratic. idiosyncrasy - idio (idiot). and idiot's ideas (idio) are never in sync [or are crazy (crasy)] with that of a normal person. thus eccentric, odd in nature!