The Collaborative International Dictionary of English v.0.48:

Cacoethes \Cac`o*["e]"thes\, n. [L., fr. Gr. ???????? of ill
   habits, ?? ???????? an ill habit; kako`s bad + ? habit]
   1. A bad custom or habit; an insatiable desire; as,
      caco["e]thes scribendi, "The itch for writing". --Addison.
      [1913 Webster]

   2. (Med.) A bad quality or disposition in a disease; an
      incurable ulcer.
      [1913 Webster]

WordNet (r) 3.0 (2006):

cacoethes
    n 1: an irrational but irresistible motive for a belief or
         action [syn: mania, passion, cacoethes]