Top Definition
A visible stain of fecal matter found in the unmentionables of those with a loose constitution.

Common among individuals experiencing Montezuma's revenge, Aztec two-step, or the Hershey squirts, and an integral feature of Louisiana Swamp Ass

A Skid Mark.
I had to throw away my grundies after a particularly nasty bout of mud butt. The ginger kisses wouldn't come out, not even in a boil wash.
by miniGMgoit July 22, 2010

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