When you disarm someone, you take their weapons away from them. An archery instructor might, for example, disarm a student if he wasn't handling his bow and arrow carefully.
To disarm is to remove a gun, knife — or any kind of weapon — from someone's hands, or from a group of people. A government might work to disarm a rebel group, for example, and police officers sometimes offer to pay money for guns in an attempt to disarm citizens. You can also use the verb to mean "win someone over," like when you disarm a cranky store cashier with your friendly smile. Disarm has an Old French root word, desarmer.