Kill 50 enemies by punching or kicking. You can punch with your left hand at any time, and with your right when you are not carrying a weapon, but can kick any time.
These are the weakest attacks, so either work on this when your current weapon breaks and before you pick up another, or try and get it done in the early stages that have weaker enemies. You can also whittle an enemy's health down with a weapon, and finish it with a kick.
Your kill counts carry over from each playthrough, so this will eventually unlock this as long as you use unarmed attacks when possible.