Someone who inadvertantly shits
their pants during a time period which it is highly uncommon for one to soil themselves.
The phrase is usually used behind the soiler's back to make fun of them or to publicly humiliate them without thier common knowledge.
Guy #1: "Dude, that chick got so wasted last night, she passed out and shit her pants"
Guy #2: "No way! What a Shittypants!"
Guy #1: "She even left little shits in the matress, and she doesn't think anybody knows she shit herself!"
Shittypants: "You guys talking about me?"
Guy #2: ........no.