Pisser dans sa culotte

Literally: To piss in one's trousers

Example: “Son imitation du prof était tellement drôle qu'on a failli pisser dans notre culotte.” — His impression of the teacher was so funny we nearly laughed our heads off.

English equivalent: To laugh one's head off

Origin: Two senses that look contrary and are in fact one: both come down to losing control of the sphincters. The phrase dates from the 16th century, though what it describes plainly predates it. Laugh hard enough with a full bladder and the abdominal contractions will do the rest, which is generally sufficient to end the laughing fit; that is the first sense, and it survives whether or not the accident actually occurs. A bad enough fright produces the same relaxation and therefore the same result, which is the second. There is a nice asymmetry in how French treats them: for fear the usual phrase is chier dans son froc instead, because urinating involuntarily can be attached to laughter without much shame, while the other thing is judged as harshly as the fear itself.