Whence the connection between peeing or having to pee, or piss the substance itself, and anger?
Pissed off. Pissy mood. Full of piss (with or without vinegar).
I suppose that if you view anger as an emotion that must be “gotten rid of” by dumping it out in some expressive fashion, it could be metaphorically described as being akin to needing to pee, i.e., if you don’t the need grows worse and you get less and less picky about when and where. Is that it? (Seems like a real stretch but it’s the best I can come up with).
I reckon as how if I were to find myself being pissed on I’d be in a severely nasty and violent mood, but the nomenclature doesn’t seem to support it. People do say “pissed” but if there’s a directional adverb it is always “off”, not “on”.
I guess if I wet my pants I would also not be in the best of moods either, and you could describe me as “pissy” or “pissed”, but the mood would run more towards gloom and misery and humiliation, not anger (although irritability is a possibility).
Then there’s the British interpretation of “pissed”, which pertains to having consumed enough alcohol to be fupped duck, which not only causes one to piss a lot but also, with some personalities at any rate, leads to increased likelihood of violent acts. (Me, I just get silly and then go to sleep). A distinct possibility, I suppose.
The archives indicate that Cecil has not addressed the subject. Any vulgar etymologists care to step up and suggest an origin?