Source : Webster's Revised Unabridged Dictionary (1913)

Spit ball \Spit ball\ (Baseball)
   A pitched ball in throwing which the pitcher grips the ball
   between two, or three, fingers on one side (which is made
   slippery, as by saliva) and the thumb on the other side, and
   delivers it so that it slips off the fingers with the least
   possible friction. When pitched directly overhand a spit ball
   darts downward, when pitched with the arm extended sidewise
   it darts down and out. [Cant] -- {Spit baller}.