It's is a British term that people often use to say 'are you joking' or 'are you serious'. 'Piss' means 'urine'. Someone who takes the piss a lot is often called a 'piss take' since they 'take the piss'.
A:sorry I ain't got your money today mate

B: are you taking the piss!?
by Light skin goddess January 18, 2018
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