Green’s Dictionary of Slang

the posterior, the buttocks.

1785, 1788, 1796		Grose Classical Dict. of the Vulgar Tongue n.p.: Douglas, Roby Douglas, with one eye and a stinking breath, the breech.
1811		Lex. Balatronicum.
1823		Egan Grose’s Classical Dict. of the Vulgar Tongue.
1980		Maledicta IV:2 (Winter) 198: If he is not particular as to whether he gets her ’twixt wind and water, he may prefer […] to get stuck into her Rory [sic] Douglas — that part which has one eye and a stinking breath.
1998		R. Puxley Fresh Rabbit 94: An ancient expression for that part of the anatomy [...] is Roby Douglass.