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)