When you piss or poo yourself because there are no pissing/pooing facilites located nearby. Generally there are two choices. Find a quiet spot and relieve yourself against a wall/parked car or if wearing dark trousers, run the risk of relieveing yourself in your pants.
Before the excretement occurs the individual can suffer large amounts of panic/anxiety before the event and embarrasment and disdain after the event, especially if witnessed by passers-by. If one manages to avoid getting caught short then the individual can enjoy a warm, triumphant feeling of relief and accomplishment.
Also getting "caught short" is a phenomenon largely related to binge drinking and is not gender specific!
Man 1: Mate are your trousers wet? Have you pissed yourself?
Man 2: Yeah I know - I got caught short ten yards away from the toilet. Nobody caught me in the act though as I faced the wall and pretended I was texting a friend! Complete fucking gayness but I got away with it!!