Green’s Dictionary of Slang

(US Und.) a safe-cracker.

1931		Irwin Amer. Tramp and Und. Sl. 36: boxer.– [...] A safe-robber.
1960		(con. 1950-1960) R.A. Freeman Dict. Inmate Sl. (Walla Walla, WA) 16: Boxer – one who specializes in cracking petes.