Fat lip or Fat lipping happens when a couple is interupted while making out, either by a room mate, a parent, etc, and one person gets hit in the mouth with an elbow.
Dude, I was totally making out with her, and then my Bro walked in and she totally gave me a Fat Lip
When a man or a woman, who has recently had a close encouter with posions ivy, performs cunnilingus on a woman. A sudden swelling in that region is known as the FAT LIP.
fat lip
This also works with heavy petting. Her labia lips will swell. A la, the fat lip.
Teddy came home with poision ivy last night, and gave Tina a fat lip.