knuckle ball

or knuck·le·ball

noun Baseball.
  1. a slow pitch that moves erratically toward home plate, usually delivered by holding the ball between the thumb and the knuckles of the first joints of the first two or three fingers.

Origin of knuckle ball

An Americanism dating back to 1905–10
Also called knuck·ler [nuhk-ler] /ˈnʌk lər/. Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2018

Word Origin and History for knuckleball

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.

Online Etymology Dictionary, © 2010 Douglas Harper