A knuckle-duster is a piece of metal that covers the knuckles of the hand. A person slips it on so that a punch hits much harder, and can cut skin or break bone. It is a weapon, not a tool, and in many places it is against the law to carry one. Knuckle-duster is the usual British name; American English more often says brass knuckles. You meet the word in crime stories, news about fights, and police reports, not in polite everyday talk.