When going down on someone and you eat some of their flatulence. Must feel the air move across your lips for it to count.

Swamp biscuit with gravy: when you have a swamp biscuit that is accompanied by stool loose or formed
"Yo los, I was eating this girl out and she gave me a swamp biscuit."
Los says, "b, did it have gravy?"
B says "naw man no gravy this time maybe next sounds delicious!"
by The Oh So Easy June 6, 2015
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