The action of slapping your junk/balls against your thighs to produce a sound.
Roommate 1: Dude, what the hell was that noise from your room last night!?
Roommate 2: Just me standing in front of the mirror slappin' out that flappywappins.
when you're holding up your phone and making faces at it, as though you are taking a selfie, but you're really taking a picture of the person across from you or the wall or anything else that seems interesting but you don't want to be caught dead taking a picture of.
This action is often made more convincing by wiggling the eyebrows or opening the mouth, to pretend you're trying to get a Snapchat filter to work.