noun
(baseball) A pitch thrown with the ball gripped on the fingertips and released with no rotation, which travels over an unpredictable path to the plate due to micro-turbulence in the air.
(cricket) A type of delivery where a fast bowler holds the ball on the knuckles of the index and middle fingers.
(figurative, informal) An event that is hard to predict.
verb
(intransitive, baseball) To pitch knuckleballs.