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)
by Roberto y Maria August 19, 2011
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