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2 definitions by nate fingerman

 
1.
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...
i gave my roommate's mom the underbite last night.
by nate fingerman July 16, 2006
 
2.
1. while making out with a chick, you slide your hands down her pants and put your digits in her vagina.

2. reverse underbite with penetration.
my date was going bad until i gave her the intruder. bitch was loving it, yo.
by nate fingerman July 16, 2006