The arm of a chair is the part on which you rest your arm when you are sitting down.
(椅子的) 扶手 • Mack gripped the arms of the chair. 麦克抓紧了那把椅子的一对扶手。
If you arm someone with a weapon, you provide them with a weapon.
武装 • She'd been so terrified that she had armed herself with a loaded rifle. 她是如此害怕,以至于用一支上了膛的步枪把自己武装了起来。