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Pants that are worn so often, and are so comfortable, that they've been sweat in, shat in, and have soaked up swass on multiple occasions.

Could also be described as sweat pants on steroids.
Scott: Dude, you just spilled your nachos and beer ALL over your pants!
James: No worries bro, these are my shwat pants, they've seen a lot worse!
by JettSkyler April 15, 2011