bucker

A horse or other animal that bucks.

A holder-up; one who bucks rivets, typically holding a heavy bucking bar against the bucktail of a rivet which is heated if necessary till it is soft, while the riveter (or gunner or, before mechanisation, basher) uses a rivet gun (an adjustable pneumatic hammer) fitted with a rivet set, against the factory head to provide impulses which upset the bucktail into a field head.

One who bucks ore.

A broad-headed hammer used in bucking ore.