Green’s Dictionary of Slang

piss off v.

[piss v. (1)]

1. [1910s+] (also piss out) to leave.

2. [1950s+] (also piss up) to annoy; thus pissed-offness, a state of anger; thus euph. as wee-wee off v.

3. in trans. use of sense 1, to send someone away.