snorting the foam from your beer. typically from the foam formed after a fresh pour at the top of the solo cup of natty light. this is done to give you a little alcohol buzz and fulfill the physical desire of snorting drugs without actually snorting illegal drugs. foam domes are also useful to reduce the foam at the top of your cup instead of waiting for it to dissipate on its own. doing a foam dome is the act of plugging one nostril, dipping your nose into the foam, and snorting it, the same way one would snort powder drugs.
When you nut in a girl's mouth and she continues to suck until well beyond the point of feeling good so you slap the back of her head so she foams out the mouth.