RhymeZone: curveball definitions

Definitions of curveball:

noun: (baseball) A forespin pitch thrown by rotating the index and middle fingers down and resulting in motion down "curve"
noun: (by extension, chiefly US) An unexpected turn of events initiated by an opponent or chance; an exception or outlier.
verb: (baseball) To throw a curveball.

(Definitions from Wiktionary)