What is Ass Slap?
A pat of the butt, used by male athletes used to signify a good play/move.
Joey ass slapped me after I hit a homerun.
The act of slapping one's ass, often in conjunction with one or more ass(es); an insult often derived by pulling one's pants down, instructing someone to place their face vertical to the ground and within the vicinity of exposed ass, and summarily having it slapped by said ass.
Your so stupid, you deserve an ass slap.