When you squirt mustard (peanut butter works too) on your erect dick (flaccid works too) and come down on the fore head or cheek of your passed out brosky. Before, during, and after the event, you and your awake broskies must chant "Mustard DICK slap! Mustard DICK slap!" and record it with a phone or some shit like that.
Everybody: "Mustard DICK slap, Mustard DICK slap!"
(SLAP)
Everybody: "Mustard DICK slap!"
Victim: "What the FUCK!!!"
Everybody: "Mustard DICK slap, Mustard DICK slap..."
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.
FRIEND A: "Did you just take a stealthie of me?"
FRIEND B (turning phone around): "no I was just using snapchat's new filter, see?"