What is "bitter apple"

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.