This is a word used by construction workers as shorthand for a sheet metal worker
For example, the tin knockers installed the ventilation system in the new women's shelter.
Construction slang for a sheet metal worker. A trade far superior to pipe fitting. Only union sheet metal workers can be called this, non union workers are just called dirty rat scab bastards. The only true craftsmen on the job who make what they install... duct work. They are also sick individuals with a twisted world perspective. Also huge dicks. You can see them from space.
That damn tin knocker ran my apprentice off AND shit in my lunch bucket!