from Wiktionary, Creative Commons Attribution/Share-Alike License
- v. To tease, ridicule or mock (someone).
- v. To subject those present to teasing, ridicule or mockery, or to show contempt.
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, usu. 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. (Wiktionary)