piss (one's) pants
1. Literally, to urinate while still wearing one's clothing. I need you to pull over the car right now, or else I'm going to piss my pants!
2. To laugh in a hysterical or uncontrollable manner (i.e., to the point where one might actually urinate in one's pants). Wow, that standup comedy was great! She had the audience pissing their pants!
3. To be absolutely petrified with terror; to be scared out of one's wits. I was pissing my pants when that huge burly guy started threatening me and my girlfriend.