The phrase "brass knuckles" refers to a type of weapon that covers the knuckles, usually made of metal, which provides additional strength when punching someone. Full definition
Police claim the van's driver admitted that he armed himself with brass knuckles after being struck in the head by the woman with a water bottle. (hernandoinjuryattorney.com)
In another, he sucks liquid through a tube, then removes a knife attached to a set of brass knuckles from a watermelon, punches himself in the face, and subsequently spits out the liquid as if it were blood. (dailyserving.com)
Well, they forgot that a lot of States brand brass knuckles as illegal. (thatvideogameblog.com)