piss (in) one’s pants v.
1. [1930s+] to be terrified (cf. pee (in) one’s pants under pee v.).
2. [1920s+] (US) to be very excited, in a state of suspense.
3. [1950s+] to be irritable or angry.
4. [1960s+] to laugh hysterically.
5. [1970s+] to make a fool of oneself.
6. [1990s+] to be very keen.