If legal definition is your thing, here’s an article that cites a number of legal cases affirming that brass knuckles do not constitute legally protected “arms” in relation to the second Amendment.
One of the earliest is “English v. State”, 1871. The court-affirmed argument against such weapons’ supposed protection under the Second Amendment reads in part
No kind of travesty, however subtle or ingenious, could so misconstrue this provision of the constitution of the United States, as to make it cover and protect that pernicious vice, from which so many murders, assassinations, and deadly assaults have sprung, and which it was doubtless the intention of the legislature to punish and prohibit. The word “arms” in the connection we find it in the constitution of the United States, refers to the arms of a militiaman or soldier, and the word is used in its military sense. The arms of the infantry soldier are the musket and bayonet; of cavalry and dragoons, the sabre, holster pistols and carbine; of the artillery, the field piece, siege gun, and mortar, with side arms.(p.477)
The terms dirks, daggers, slungshots, sword-canes, brass-knuckles and bowie knives, belong to no military vocabulary. Were a soldier on duty found with any of these things about his person, he would be punished for an offense against discipline.