Green’s Dictionary of Slang

1. (US black) an authoritarian figure.

1934		AS Dec. 288: Hammerman. Generally, anyone with more authority than the speaker.
1970		C. Major Dict. Afro-Amer. Sl. 63: Hammer-man An authoritarian figure.

2. (Aus.) an auctioneer.

1996

J. Byrell Lairs, Urgers & Coat-Tuggers 184: [I]t looks as if the hammerman is going to dud a sale and Zelica will end up being used by anklebiters to glue cut-out pictures in their school books with.