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/.
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2018