when you're making out with a chick, and you slide your hands down her pants and pinch her bare ass. UNDERBITE!!! see the intruder...

when your eating pussy and you get a mouthful of shit

Someone who has a major underbite with crooked teeth. And chia pet hair.

When an inbred looking girl without teeth is going down on you and then sucks your balls from below, not under, like a fish to bait.

Severe underbite -- when it smells like pussy but it tastes like shit.