knuckleball (n.)

also knuckle-ball, baseball pitch, by 1909, from knuckle (n.) + ball (n.1). So called from the position of the fingers in throwing it. Short form knuckler attested from 1914 (earlier this was the name of a type of toy marble, 1895, and slang for "a pick-pocket," 1834). Related: Knuckleballer.