Okay, so I had to go and look it up.
"There is no American translation of the phrase ‘taking the piss’. If an American were to vocalize some of the things an English person feels free to express, we’d call it ‘being an asshole.’ Taking the piss is akin sarcasm, but usually far less mean. It can also mean ‘to take advantage of’ but the former is the interpretation I’ve experienced the most."
Blimey, you Brits.