1.

The rubber residue that remains on the road after someone have either accellerated or braked hard with a wheeled vehicle.

2.

The residue of feces that one might find in the bottom of the toilet bowl after someone inconsiderate have taken a dump, and not cleaned up after themselves.

"Skid mark" is a colloquial reference to a residue of feces left on the inside of a pair of soiled underwear.

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