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1.
pants with poop inside, pants with shit inside;
poopants! - just an exclamation like "shit"
by Mantal January 12, 2007
 
2.
Someone who has pooed his or her pants. Can be identified by a familiar stinkiness and an awkward guilty expression.
"Ew you stink. You're a poo pants!"
by Cartoonist June 05, 2012