Green’s Dictionary of Slang

drain v.

to urinate; usu. in combs. below.

1949

Monteleone Criminal Sl. (rev. edn) 75: drain To urinate.

In compounds

drain-off (n.)

an act of urination.

2000

J.J. Connolly Layer Cake 59: Need a good drain-off before I get in the motor.

In phrases

drain one’s/the (main) vein (v.)

(orig. Aus.) to urinate.

1967–8		Baker et al. CUSS 218: Vein, drain the Urinate.
1989		P. Munro Sl. U.
2001		OnLine Dict. of Playground Sl. 🌐 Drain the main vein v. to urinate in a particularly speedy manner (probably to avoid being caught doing it in a public place), e.g. in a shop doorway or telephone box on the way home from the pub.
2013		F. Bill Donnybrook [ebook] ‘Smells like some sour son of a bitch drained his vein back here’.

drain the dragon (v.) (also drain the lizard, …snake) [dragon n.⁴ /snake n.³ ]

to urinate.

1963–74		B. Humphries Barry McKenzie [comic strip] in Complete Barry McKenzie (1988) 30: Just hang while I nick out the gents and drain the dragon.
1989		P. Munro Sl. U.
1991		Age (Melbourne) 7 Dec. 130/5: Australia has much of which to be proud [...] drain the dragon, syphon the python, (point percy at the porcelain), having a liquid laugh down the great white telephone.
2000		P. Howard Miseducation of Ross O’Carroll-Kelly (2004) 75: I tell her I have to drain the snake. [Ibid.] 195: I tell the girls I need to drain the lizard.
2015		S. King Finders Keepers (2016) 29: Pull in there [...] I need to drain the dragon.