When one shits (the mud) and claps/shakes their ass at the same time (the flapping).
Uh oh… Jeff was the first one to fall asleep at the sleepover. Guess we’re going muddflappin’ boys! Hahahahaha, he’s gonna be so mad when he wakes up with poop all over himself!
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.