To eliminate a hard-on (go limp) by peeing instead of ejaculation. This is disappointing to men, so they don't want to go to the bathroom and piss off their hard-on. Then then say they are pissed off, or angry about it.
It has become slang for making someone angry or when something or someone makes a person angry.
Today, women have picked up on this term also, using it as getting angry (but not about their hard-on going away).
I love my errection, but when I go pee it will piss off (original usage).
He is going to piss me off (modern).