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When the skidmarks on your underwear dry out and start flaking off, turning into dust. It happens when people do not wipe up well after going to the bathroom, and when they do not change their underwear frequently.
I haven't changed my underwear in six weeks and now I think I'm starting to see some crust dust coming out of my pant leg.
by Chigglywiggly December 07, 2009
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