If something is sidesplitting, it is not a little bit funny. It is the kind of funny that makes you laugh so hard your body hurts. English has an old picture for this: you laugh until your sides — the left and right of your belly — split open. Nobody’s body really splits. It is just a loud way to say “extremely funny.” You use it about jokes, films, stories, and comedians, often in writing: a sidesplitting comedy. It is informal but not teen slang; a review can say it. Though actually, the word is too big for a quiet smile. If people only chuckled, sidesplitting is doing too much work.