Crackin' Off
To stroke your own penis repeatedly until you skeet, unless your mother comes in, then you become a serial killa.
Kid 1: He's been in the bathroom forever! What could he be doing?
Kid 2: Probably crackin' off!
Kid 1: Dude, that's not even an expression.
Kid 2: Probably crackin' off!
Kid 1: Dude, that's not even an expression.