The Collaborative International Dictionary of English v.0.48:

spitball \spit"ball`\, spit ball \spit" ball`\, n.
   1. Paper chewed, and rolled into a ball, to be thrown as a
      missile; -- a childish prank.
      [1913 Webster]

   2. (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; -- also called
      spitter. When pitched directly overhand a spit ball
      darts downward, when pitched with the arm extended
      sidewise it darts down and out. The use of such a pitch is
      against the rules of professional baseball. [Cant] --
      Spit baller.
      [Webster 1913 Suppl. +PJC]

WordNet (r) 3.0 (2006):

spitball
    n 1: a projectile made by chewing a piece of paper and shaping
         it into a sphere
    2: an illegal pitch in which a foreign substance (spit or
       Vaseline) is applied to the ball by the pitcher before he
       throws it [syn: spitball, spitter]

Moby Thesaurus II by Grady Ward, 1.0:

36 Moby Thesaurus words for "spitball":
   bowl, cast, change of pace, change-up, chuck, chunk, curve,
   downcurve, fastball, fling, flip, forward pass, heave, hurl,
   incurve, knuckleball, lateral, lateral pass, lob, outcurve, pass,
   peg, pitch, put, screwball, serve, service, shot-put, shy, sinker,
   slider, sling, spitter, throw, toss, upcurve