Green’s Dictionary of Slang

an act of urination.

1835		‘Meat and Gravy’ in Fanny Hill’s Bang-Up Reciter in Spedding & Watt (eds) Bawdy Songbooks (2011) III 316: But first you must make a small drain in de pot. / The blowen stoop’d down in a shake to serve him, / And the chamber-pot she fill’d right bnang to the brim.
1938		G. Greene Brighton Rock (1943) 237: I’m goin’ to have a drain.