knuckleball

Short definition from Wiktionary

Noun

1. 1
   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.
2. 2
   A type of delivery where a fast bowler holds the ball on the knuckles of the index and middle fingers.
3. 3
   An event that is hard to predict.

Verb

1. 1
   To pitch knuckleballs.