Green’s Dictionary of Slang

extract the urine v.

(orig. milit.) euph. for take the piss (out of) under piss, the phr.

[Aus]Register 8 Apr. 🌐 ‘On the four days there actually WERE lectures?’ ‘Mmmmm.’ ‘And you don’t think that you’re extracting the urine?’ ‘What?! It was hard work!’.