To slap your penis into someones face or somewhere else. Usually happens when everyone is drunk or stoned and someone passes out or falls asleep. When he/she gets zwaffle'd, everyone laughs their ass off.
Guy 1: Look, he passed out hahahha.
Guy 2: Well go ahead, give him a zwaffle
Guy 1 *slap*
Guy 3: w..w w-What the fuck man?