take the piss

Etymology:

• Possibly from piss-proud. Figuratively, to be piss-proud is to have false pride, thus taking the piss out of is to deflate their false pride, usually through disparagement or mockery. As the piss-proud metaphor became dated, taking the piss out of someone came to refer to disparagement or mockery itself, regardless of the pride of the subject. Eventually the shortened, intransitive form taking the piss became common.