Green’s Dictionary of Slang

indorse v.

[SE in + dorse, the back]

to sodomize.

[UK]‘Bumper Allnight. Esquire’ Honest Fellow 184: I quickly came home / On an English duke’s bum, / Who was fond of a fundament hump, / His master of horse / He used to endorse.
[UK]Farmer & Henley Sl. and Its Analogues.
[UK]Partridge DSUE (1984) 597/1: C.18.