Flatulence which loosens the bowels enough to void an amount of fecal matter, be it just a bit or an entire load. In other words, when you mean to fart and accidentally shit yourself.
Aww, man... I just let a hard fart go, and now I've got to throw out my new underwear!
You should check your drawers, man. That sounded like a hard fart to me.
The forcing out of flatulence in the hopes of either encouraging a bowel movement or creating a loud noise to impress your friends. Hard farts can be very dangerous in that they can be followed by moderate to severe pain that can either be temporary or more chronic as in the case of hemorrhoids. They can also result in the soiling of oneself and creating and embarrassing situation
"Johnny pushed a loud hard fart out. Everyone laughed. Johnny abruptly left the party, never to return"