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.

When you accidentally drip mustard off of a pretzel onto your crotch.

When slap with dick mustard

When you are so drunk you jizz and piss at the same time.