Green’s Dictionary of Slang

dubber n.1

[dub v.1 ]

1. (UK Und.) a thief who specializes in picking locks.

[UK]B.E. Dict. Canting Crew n.p.: Dubber c. a picker of locks.
[UK]J. Hall Memoirs (1714) 5: Dubbers, Such as rob Dwelling-houses, Ware-houses, Coach-houses or Stables, by picking the locks thereof.
[UK]A. Smith Lives of Most Notorious Highway-men, etc. (1926) 205: [...] Dubber, a picker of locks.
[UK]New Canting Dict. [as cit. c.1698].
[UK]Bailey Universal Etym. Eng. Dict. [as cit. c.1698].
[UK]Grose Classical Dict. of the Vulgar Tongue.
[UK]G. Andrewes Dict. Sl. and Cant.
[UK]Lex. Balatronicum.
[UK]Egan Grose’s Classical Dict. of the Vulgar Tongue.
[UK]G. Kent Modern Flash Dict.
[UK]Flash Dict. in Sinks of London Laid Open.
[UK]Duncombe New and Improved Flash Dict.
[US]Matsell Vocabulum.

2. a turnkey, a gaoler.

[UK]Lex. Balatronicum n.p.: How the dubber served the cull with hung beef; how the turnkey beat the fellow with a bull’s pizzle.