Green’s Dictionary of Slang

(US Und.) a burglar.

1897		Eldridge & Watts Our Rival, the Rascal 99: The ‘flat worker’ is a cunning thief who usually makes his skeleton keys to fit any ordinary lock.
1901		C.R. Wooldridge Hands Up! 330: She said that Delia Foley and George Mead [...] were flat workers, burglars and thieves.
1910		J. Sullivan ‘Criminal Sl.’ in Amer. Law Rev. LII (1918) 890: A second-story worker who breaks and enters dwelling houses is called a ‘houseman,’ ‘porch climber’ and ‘flat worker.’.
1924		G. Henderson Keys to Crookdom 405: Flat worker. One who robs flats, house-breaker, prowler.
1949		Monteleone Criminal Sl. (rev. edn).
1950		Goldin et al. DAUL 71/2: Flat-worker. A small-time burglar who robs flats and apartments when occupants are absent.
1970		R. Fabian Anatomy of Crime 193: Flatworker: Specialist in robbing flats.