To strike the male genetalia quickly and unexpectedly with the back of ones hand, using a whipping motion. This is usually follwed by laughing and pointing.
Striking of the scrotum is more effective than striking of the penis.
So here I was at this party trying to spit game to this girl and Jeff walks by and jimmytaps me hard as shit. Needless to say, after I got up, I chased him down and beat the shit out of him.