The Collaborative International Dictionary of English v.0.48:

Colocynth \Col"ocynth\, n. [L. colocynthis, Gr. ?. Cf.
   Coloquintida.] (Med.)
   The light spongy pulp of the fruit of the bitter cucumber
   (Citrullus colocynthis, or Cucumis colocynthis), an
   Asiatic plant allied to the watermelon; coloquintida. It
   comes in white balls, is intensely bitter, and a powerful
   cathartic. Called also bitter apple, bitter cucumber,
   bitter gourd.
   [1913 Webster]