Green’s Dictionary of Slang

let-in n.

[let in v.]

1. a hoax, an act of cheating.

[UK]W. Perry London Guide.
[UK]J. Manchon Le Slang.

2. a robbery.

[UK]J. Manchon Le Slang.

3. an illegal victimization.

[UK]J. Manchon Le Slang.