A piece of toilet paper inserted between ones butt cheeks to keep sweat and/or crap stains from reaching ones underwear
Butt Rag (aka ass tampon). After taking a dump and wiping clean, you tear off 2 to 3 squares of toilet paper, fold them on top of each other to form a square, then fold that in half, then (optional) you apply some baby or talcum powder to the outer edge. Then wedge it between your ass cheeks in the crack. Keeps your underwear nice and skid free until the next dump.