Green’s Dictionary of Slang

of a woman, allegedly to be working as a prostitute.

1678		J. Ray Proverbs (2nd edn) 90: She’s as common as a barbers chair. As common as the highway. She lyes backwards and lets out her fore-rooms.
1696		Motteux (trans.) Gargantua and Pantagruel (1927) II Bk IV 468: Is there anything of the feminine gender among them? [...] Will they lie backwards, and let out their fore-rooms?
1785, 1788, 1796		Grose Classical Dict. of the Vulgar Tongue n.p.: Room. She lets out her fore room and lies backwards: saying of a woman suspected of prostitution.
1811		Lex. Balatronicum [as cit. 1785].
1823		Egan Grose’s Classical Dict. of the Vulgar Tongue.
1823		‘Jon Bee’ Dict. of the Turf, the Ring, the Chase, etc. 133: She who is said to ‘let out her parlour and lie backward’ cannot be supposed to repose with her face downwards.
1833		‘The Amiable Family’ in Fal-Lal Songster in Spedding & Watt (eds) Bawdy Songbooks (2011) III 8: My wife Sal, who never does lack words, / Took in tenants, but they prov’d such blackguards — / She did say, by gole! / She let out her whole / Of her house, and then lay herself backwards .
1870		Cythera’s Hymnal [as 1833].