take the piss (out of) (someone or something)
To tease, mock, or ridicule (someone or something); to joke or kid around (about someone or something). Primarily heard in UK, Ireland. If you are so serious that you can't take the piss out of yourself every once in a while, you're going to have a hard time enjoying most of life. It really hurt Steph's feelings to know that the group had been taking the piss out of her that whole time. Brian was a bit of a troublesome student and tended to take the piss whenever class began.
take the piss out of
Chiefly British To tease or mock (someone).