Source: And Introduction to British Artillery in North America. Museum Restoration Service, All of, Ontario. 1972. S. James Gooding. p. 24-25.

That's a really good question! I'm not too sure, but I'll tell you how it relates to artillery; "The vent on the earliest cannons were merely a hole drilled through the metal from the outside to the bore, but beginning about 1812, experiments were begun on the installation of a bushing which could be removed from the gun as it became worn and replaced with a new one without a great deal of time and effort been expended. This proved to be quite successful and copper was first introduced as the metal for the bushing material." So, artillerymen who were selected to replace the bushings were often seen, with their hammer and tools, as men who were ‘whacking at the bushing’; Bush Whackers,. someone working feverishly to get something out and then to put something new back in again as quickly as possible.