The Brass Knuckles were added to multiplayer in the 1.04 update. They are available from the Melee class, and must be earned through Supply Drops. Like other melee weapons, they can achieve a one-hit-kill on an enemy at full health. It is functionally identical to the other standalone melee weapons.
Zombies[]
Floyd Campbell is seen equipping a brass knuckle in the intro cutscene of Shadows of Evil, but it is not available in this game mode.