Green’s Dictionary of Slang

1. (also horizontalist) an up-market prostitute, a kept woman (Lorette in cite 1909 refers to the area of Notre Dame de Lorette, Paris IXeme, a centre of commercial sex in 19C).

1909		J. Ware Passing Eng. of the Victorian Era 154/2: Horizontal (Anglo-Fr., 1886). Lorette.
1954		M. Bodenheim My Life and Loves in Greenwich Village (1961) 94: They call themselves models – in the Village we call them horizontalists: they make their living lying down.

2. (US, also the horizontals) sexual intercourse; usu. as do a horizontal; note extrapolation in cit. 1950.

1833		‘The Amiable Family’ in Fal-Lal Songster in Spedding & Watt (eds) Bawdy Songbooks (2011) III 8: An horizontal was once her delight now, / She the fingering did it all right now, / But the young wretch does say. / To me every day, / ‘I wants for to have an upright now!’ .
1870		Cythera’s Hymnal 56: A horizontal was once her delight now, / She can do all the fingering right now, / But the young bitch does say. / To me t’other day, / ‘I should like for to have an upright now!’.
1896		Farmer Vocabula Amatoria (1966) 139: Foutre en espalier = to copulate standing; ‘to do a horizontal’.
1934		(con. 1920s) J.T. Farrell Young Manhood in Studs Lonigan (1936) 389: Besides, the black boys were happiest when engaged in the horizontals.
1950		K. Amis letter 12 June in Leader (2000) 233: Two of my girl pupils told me the other evening that I am alpha-plus for women, and that they wish I wasn’t married, so you see what a strain it is to stay vertical with them.