Green’s Dictionary of Slang

piss oneself v.

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

1. [late 18C; 1960s+] lit. to urinate on oneself.

2. [1960s+] to be utterly terrified, whether lit. causing urination or not.

3. [1940s+] to laugh uproariously; usu. the urination is just fig.