The word is a combination of "shit" and "fart"---you get the idea. It's when you let out a massive fart and end up with some hershey stains on your drawers/panties.
when you needa do a smelly as ripper and then you push a bit too hard and you do a little pellet in your pants. (sometimesnot that little). can come in great amounts, depends on how hard you push.
"I was bored in class and i cut the cheese"
" i can smell that... it smells like more than a fart"
"yeah i just sharted.."
"OH YOU FUCKEN SMELL MAN GO HAVE A WASH"
"no i like the warm sensation trickling down my leg"
"oh dude im gonna puke"