Flashlight

A strong flashlight is a very strong deterrent. Imagine wanting to rob someone and they suddenly blind the sh*t out of you. Now you're basically at their mercy. If they're using a high quality metal flashlight, they could absolutely smack you really hard with that. Some flashlights even have spiky edges.

Of course, you could wield a flashlight in one hand (to keep your opponent blinded) and beat them up with your other hand.

Flashlights also happen to be legal, which is a great bonus.