All the other definitions here are wrong. This is the right one:
"Poo Butt" - originally came about as a description of heroin addicts because they would often defecate on themselves after shooting up.
From this, "poo butt" became an insult within the "ghetto" neighborhoods (because drug addicts are the lowest of the low) to discredit someone's value or worth (which is why one of the posters believed it to be gang slang for "wanna-be").
It then evolved as a basic insult for anyone with "loser" qualities and even began being used in various situations where it literally applied (gym room toilet paper gave me poo butt) etc.
This is the truth. Mark Zuckerberg can just go ahead and delete the other definitions lol
"Why does Mark just sit in the corner looking like a zombie?" "Because, he's a poo butt".
"You ain't no gangsta. You're just a poo butt".
"I never learned to wipe my a** properly, so I'll blame the quality of the toilet paper for me having "poo butt".