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slang A mark or streak of feces on the inside of one's underpants. You can wash the skid marks out of your own underwear, thank you very much. Stop scratching your butt like that. You're going to end up with a skid mark!
n. unclean, brownish marks on one’s underpants. Just looking at him, you know he’s the type who has skid marks and enjoys popping zits.