This is British slang used amongst teenagers. It's a way of saying getting wasted or drunk. It just means they're going to buy some cheap alcohol and get drunk off their faces.
Chav 1 - What ya doing this weekend mate?
Chav 2 - Getting steamed at this sick party, bro. You?
Chav 1 - Safe. I'm going swimming.
Chav 2 - Getting steamed at this sick party, bro. You?
Chav 1 - Safe. I'm going swimming.
by Popsicle21 October 10, 2009