A fist wearing brass knuckles.
brass knuckles definition by Webster's New World
Webster's New World College Dictionary Copyright © 2010 by Wiley Publishing, Inc., Cleveland, Ohio. Used by arrangement with John Wiley & Sons, Inc.
linked metal rings or a metal bar with holes for the fingers, worn for rough fighting
brass knuckles definition by American Heritage Dictionary
plural noun (used with a sing. or pl. verb)
A metal chain or a set of rings attached to a bar that can be fitted over the fingers to increase the impact of a blow with the fist.