Another way to say "screw it" or "to hell with it".

Piss is verb describing urinating. The yellow liquid that you release from the tip of your penis is called urine. Piss a way to describe realeasing that urine. It also describes the great feeling of emptiying your bladder

When someone gets really drunk, like where they start slurring and can't walk straight.

Drunk. Very drunk.

ANGRY, UPSET

In England the term pissing means to go bar hopping, which I believe is how we began to use it to mean "urinating"

Listen to the lyrics from ChumbaWambas Song 'Tubthumping' they are refering to bar hopping.

"I get knocked down
But I get up again
You’re never going to
Keep me down

Pissing the night away
Pissing the night away"

catagorized in "England" as being realy drunk, or by "the rest of the world" as being very angry.
this is incorect as english people have been being pissed (drunk) or being pissed at (angry/very angry) people for more than 1000 years.
the words and spelling might be differrent but the attitude, temper, emotion and disposition (depending on the state of sobriety) are the same.