Source : Webster's Revised Unabridged Dictionary (1913)

Cacoethes \Cac`o*["e]"thes\, n. [L., fr. Gr. ???????? of ill
   habits, ?? ???????? an ill habit; ? bad + ? habit]
   1. A bad custom or habit; an insatiable desire; as,
      caco["e]thes scribendi, ``The itch for writing''.
      --Addison.

   2. (Med.) A bad quality or disposition in a disease; an
      incurable ulcer.

Source : WordNet®

cacoethes
     n : an irrational but irresistible motive for a belief or action
         [syn: {mania}, {passion}]