taking the piss

'Taking the piss' is British and Irish English meaning either (1) mocking someone, or (2) acting unreasonably / pushing one's luck. 'Are you taking the piss?' = 'are you joking / being unreasonable?'. Mildly vulgar but widely acceptable in British conversation. If your teen uses it, they are using everyday British slang. Harmless among friends; would not appear in formal writing.

'Taking the piss' is British and Irish English with two related senses: (1) mocking, teasing, or making fun of someone or something ('he's just taking the piss'); (2) acting unreasonably or pushing one's luck ('£20 for a coffee? They're taking the piss'). Mildly vulgar but widely acceptable in casual British conversation across most settings. In formal English use 'mocking' or 'being unreasonable'. A common learner mistake is to use it in American English, where it is largely unknown and may shock listeners with its literal reading.