Green’s Dictionary of Slang

back v.1

[SE back, to cover or copulate]

to have sexual intercourse.

[UK] ‘The Four-legg’d Quaker’ in Rump Poems and Songs (1662) i 359: She was by thee compelled; / Poor thing, whom no man ever backt, / Thou wickedly hast Bellied.
[[UK]Rochester ‘A Ramble in St James’s Park’ in Works (1999) 79: A vast Meal of Nasty Slime; / Which your devouring Cunt had drawn / From Porters Backs, and Footmens Brawn].
[US]T. Swerdlow Straight Dope [ebook] He’s who backed Ange in her gallery. And backed her other ways as well [...] Ange was under the impression that she was going to be Mrs. Schlein.