take the piss out of someone
to let the air out of their balloon, to mock/make fun of them. Customarily, shortened to just take the piss.
From the movie Snatch:
"take the piss out of someone" is abbreviated to "take the piss"
Boris the Blade: (mutters vague insults at Tony)
Bullet Tooth Tony: Don't take the piss (i.e., out of me), Boris!
Boris: Fuck you!
Tony: (empties the magazine of his Desert Eagle .50 into Boris)
"take the piss out of someone" is abbreviated to "take the piss"
Boris the Blade: (mutters vague insults at Tony)
Bullet Tooth Tony: Don't take the piss (i.e., out of me), Boris!
Boris: Fuck you!
Tony: (empties the magazine of his Desert Eagle .50 into Boris)
Taking the piss [out of someone/something]
Making fun of someone or something, or kidding with someone about something.
You really won the lottery?
Naw, I'm just taking the piss.
Naw, I'm just taking the piss.