Green’s Dictionary of Slang

backed adj.1

[either f. lying on one’s back, or, according to B.E. and then Grose (1785), f. being supported on the backs of those who carry one’s coffin; SE backed, supported at the back, underpins the latter ety.]

dead.

[UK]B.E. Dict. Canting Crew.
[UK]A. Smith Lives of Most Notorious Highway-men, etc. (1926) 201: Backt, dead; as, he wishes the old man backt, that is, he longs to have his father on six men’s shoulders; or as, his back’s up, that is, he is in a fume, or angry.
[UK]Bailey Universal Etym. Eng. Dict. n.p.: back’ed, dead; as He wishes the Senior backed, i.e. He longs to have his Father upon Six Men’s Shoulders.
[UK]B.M. Carew Life and Adventures.
[UK]Grose Classical Dict. of the Vulgar Tongue.
[UK]Lex. Balatronicum.
[UK]Egan Grose’s Classical Dict. of the Vulgar Tongue.
[UK]Duncombe New and Improved Flash Dict.