Green’s Dictionary of Slang

backslang it v.

[? link to back slum under back adj.2 ; the term predates the linguistic variety of backslang, while there seems to be no link to slang n.1 itself; ? link to sling one’s hook v. (1)]

1. to make a deliberate detour to avoid meeting a certain person or persons.

[UK]Vaux Vocab. of the Flash Lang. in McLachlan (1964) 225: back-slang: [...] to go a circuitous or private way through the streets, in order to avoid any particular place in the direct road, is termed back-slanging it.

2. to leave by the back door, thus to leave surreptitiously, quietly.

[UK]Vaux Vocab. of the Flash Lang. in McLachlan (1964) 225: back-slang: to enter or come out of a house by the back-door [...] is termed back-slanging it.
[UK]Egan Grose’s Classical Dict. of the Vulgar Tongue.
[UK]Hotten Dict. of Modern Sl. etc.
[UK]Hotten Sl. Dict.
[UK]B.M. Carew Life and Adventures.
[UK]J. Ware Passing Eng. of the Victorian Era 14/1: Back slang it (Thieves’). To go out the back way.

3. (Aus.) to request lodgings from strangers as one travels through the back country.

[UK] E.E. Morris Austral Eng. 14/1: In the back-blocks [...] where hotels are naturally scarce and inferior, the traveller asks for hospitality [and] is always made welcome. There is no idea of anything underhand on the part of the traveller, yet the custom is called back-slanging.