Green’s Dictionary of Slang

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

c.1698		B.E. Dict. Canting Crew n.p.: Dubber c. a picker of locks.
1708		J. Hall Memoirs (1714) 5: Dubbers, Such as rob Dwelling-houses, Ware-houses, Coach-houses or Stables, by picking the locks thereof.
1719		A. Smith Lives of Most Notorious Highway-men, etc. (1926) 205: [...] Dubber, a picker of locks.
1725		New Canting Dict. [as cit. c.1698].
1737, 1759, 1760, 1776		Bailey Universal Etym. Eng. Dict. [as cit. c.1698].
1785, 1788, 1796		Grose Classical Dict. of the Vulgar Tongue.
1809		G. Andrewes Dict. Sl. and Cant.
1811		Lex. Balatronicum.
1823		Egan Grose’s Classical Dict. of the Vulgar Tongue.
1835		G. Kent Modern Flash Dict.
1848		Flash Dict. in Sinks of London Laid Open.
c.1850		Duncombe New and Improved Flash Dict.
1859		Matsell Vocabulum.

2. (also dubberman) a turnkey, a gaoler.

1811		Lex. Balatronicum n.p.: How the dubber served the cull with hung beef; how the turnkey beat the fellow with a bull’s pizzle.
1842		Flash (NY) 3 July n.p.: About four years ago [...] he ‘gave the Dabberman [sic] a holliday’ by making good his escape .