Brass Knuckles

Brass knuckles, also sometimes called knuckles, knucks, brass knucks, knucklebusters, knuckledusters, or a classic, are weapons used in hand-to-hand combat. Brass knuckles are pieces of metal shaped to fit around the knuckles. Despite their name, they are often made from other metals, plastics or carbon fibers. Designed to preserve and concentrate a punch's force by directing it toward a harder and smaller contact area, they result in increased tissue disruption, including an increased likelihood of fracturing the victim's bones on impact. The extended and rounded palm grip also spreads across the attacker's palm the counter-force that would otherwise be absorbed primarily by the attacker's fingers, reducing the likelihood of damage to the attacker's fingers.

Metal ring and knuckle style weapons date back to ancient times and have been used all over the world for many hundreds of years. The Nihang Sikhs used an early variant called Sher Panja in the 18th century. Cast iron, brass, lead, and wood knuckles were made in the United States during the American Civil War (1861–1865). Soldiers would often buy cast iron or brass knuckles. If they could not buy them, they would carve their own from wood, or cast them at camp by melting lead bullets and using a mold in the dirt.

Some brass knuckles have rounded rings, which increase the impact of blows from moderate to severe damage. Other instruments (not generally considered to be "brass knuckles" or "metal knuckles" per se) may have spikes, sharp points and cutting edges. These devices come in many variations and are called by a variety of names, including "knuckle knives."

...
Wikipedia