9 Grammatical Pitfalls
also knuckle-ball, baseball pitch, by 1909, from knuckle (n.) + ball (n.1). So called from the position of the fingers in throwing it. Related: Knuckleballer.
: a knuckleball artist
A pitch thrown from the knuckles that moves slowly and erratically; butterfly ball: Leonard's tantalizing knucklers (1906+ Baseball)