Literally it translates to 'being peeing oneself', but it's often used to express that someone is laughing so hard or is in so much suspense or fear that they could pee their pants. It's a colloquial and humorous expression.
Examples
- Spanish: Estaba tan asustado viendo la película de terror que estaba haciéndose pipi.
- English: He was so scared watching the horror movie that he was 'peeing himself'.