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2.
The sexual act where you spread your partners rectal flaps with the knuckles of your index and fore finger, then cum with the screwball on a count of two balls and one strike.
The bases were loaded so I slipped her a knuckleball. Garth, she definitely won't be walking home any time soon. Two balls, on strike, and I'm out - she was like WTF!
by Lovell from UT November 04, 2010
 
1.
A baseball pitch which follows an unpredictable path. Very difficult for batters to hit and pitchers to throw. Only a handful of pitchers in the major league actually utilize it.
Tim Wakefield Throws the knuckleball so good, it literally corkscrews.
by Lutzie October 07, 2009