Green’s Dictionary of Slang

1. (US und.) a bank robber, who uses skeleton keys that he or another has created after obtaining impression of the necessary locks.

1845		Nat. Police Gaz. (NY) 1 Nov. 83/3: The man figured in London [...] as one of the leaders of the swell mob, and was considered as the best ‘screwsman’ and ‘fitter’ in the metropolis, but was finally ‘served’ with a term in Botany Bay.
1882		A. Trumble Crooked Life in Nat. Police Gaz. 6 May 6/1: Burglars are divided [...] into three great classes. The first of these is the ‘high tobyman,’ otherwise known [...] as the ‘fitter.’ It is to this exalted rank that the bank burglar belongs.

2. (US Und.) a maker of skeleton keys.

1859		Matsell Vocabulum 32: fitter A fellow that fits keys to locks for burglars.
1895		Crowe Aus. Sl. Dict. 29: Fitter, a locksmith who makes burglars’ keys.
1959		J. Gosling Ghost Squad 26: Jack the Fitter, a maker of skeleton keys.

3. (UK Und.) a corrupt supplier who provides criminals with disguises, weapons, transport etc.

1941

V. Davis Phenomena in Crime 27: He’s a fitter [...] He fits out the ‘boys’ with anything they want.

4. (UK Und.) one who is skilled in selecting a key, chosen from a large bunch, to open a lock for their criminal accomplices.

1955		B. Hill Boss of Britain’s Underworld 82: Tosh was a fair fitter. He usually could find a key out of our bunch of a thousand from the first score he tried.
1955		B. Hill Boss of Britain’s Underworld 84: I have never met such a wizard with locks and keys as Bob the Fitter. He reckoned he could fit any lock from the mass of keys he had from the first six he chose. I never saw him fail.