Green’s Dictionary of Slang

the back of a building etc.

1960		J. Franklyn Dict. of Rhy. Sl. 116/1: round the hay-stack Round the back [...] ‘’Arf a mo. I’m jus’ goin’ roun’ the haystack,’ means to the W.C. ‘The back’ is a Cockney euphemism for the W.C.
1972		Dodson & Saczek Dict. of Cockney Rhy. Sl.
1992		R. Puxley Cockney Rabbit.
2002		B. Kirkpatrick Wicked Cockney Rhy. Sl.