Green’s Dictionary of Slang

1. a robber who relies on brute force to open a safe.

1872

E. Crapsey Nether Side of NY 17: Safe-breakers are the lowest grade of operators upon the vaunted burglar-proof receptacles [...] They rely solely upon main force both in entering a building and in working upon the safe, as they pry the first open with a ‘jimmey,’ or small hand-bar, and belabor the latter with a hammer until it falls to pieces.

2. a safebreaker who relies on picklocks and similar skills rather than on explosives.

1886

T. Byrnes Professional Criminals of America [Internet ] A gang of ‘breakers’ made many thousand dollars last winter robbing post-office and store safes in all parts of the country. Their manner of operating demonstrated that they were expert cracksmen.

3. a burglar.

[1796		Sporting Mag. Sept. VIII 301/2: Midnight breakers; / Thief, thieftakers].
1905		H. Lawson ‘The Bush Fire’ in Roderick (1967–9) II 142: Flash Jim the Breaker is lying low — blue-paper is after him.
1916		Electrical Experimenter Aug. 248/3: It was easy enough to follow the suspect [...] as he strode along. Breaker and shadow, at a distance of fifty feet apart, walked for five blocks and then the man turned quickly to the right and ran down a pair of steps.
1949		Monteleone Criminal Sl. (rev. edn).
1970		‘Metropolitan Police Sl.’ in P. Laurie Scotland Yard (1972) 321: breaker, a: a house- or shop-breaker.