Green’s Dictionary of Slang

piss (in) one’s pants v.

[fig. use of piss v. (1)]

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.