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.
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!