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.