"Pissed" vs "Pissed off"

In Australian English there has always been a distinction between "pissed" (intoxicated) and "pissed off" (angry, irritated).

This is exactly the same as the British usage.

I've noticed a trend towards the American usage where "he was really pissed" is now much more likely to mean the latter.

I have not noticed that in the UK.

Being an American, I can safely say that both are used quite often to mean the same thing: angry or irritated. "I was so pissed when he spilled coffee on my new sweater" or "it really pisses me off when she talks down to me" would be understood in America as the speaker being angry. As far as I know, we never use pissed or pissed off to mean intoxicated.

Also, it is not common to use the imperative/insult (?) piss off in America, but if you were to use it you would be quite well-understood. :)

In New Zealand, that distinction still exists quite strongly. But, like you, I have heard a few people use "pissed" to mean annoyed, but the vast majority of the time, I have only heard it in the sense of being intoxicated.

I wouldn't worry too much about it though. There are so many words in English that mean "drunk" that it wouldn't cause too much to be lost. But yes, it is a little bit of a shame. But, we cannot stop the juggernaut that is the progression (for good or for bad) of the English language.

In my experience in (western) Canada the common use tends to be pissed for drunk and pissed-off for angry/frustrated/irritated.

Surprisingly given the proximity and cultural pressure of the US the he/she was pissed [angry] isn't heard all that often. Americans visiting Canada are sometimes stumped by Canadians willingness to 'get pissed' but they catch on.

Although the expression has mostly disappeared we used to hear/say 'pissed to the gills' for drunk.