**Marks on your underwear left by shit, usually permanent. Caused by Sharting (A fart that turns out to be a shit), also caused by photo finish shits, juicy farts, not wiping completely, and most generally crapping your pants.
**Side effect- buying dozens upon dozens of new pairs of underwear.
Skid Marks are the most permanent thing on this Earth, no matter how much you scrub they stay. No matter how much bleach you dump on your underwear it doesn't even fade.
**Synonym- Racing Stripes
**Dude, did you just shit your pants? You are going to have major skid marks.
**What color where these when you originally bought them?