* armchair - a chair with armrests
* armful - the amount that can be held in one's arms
* armhole - the opening in a garment for the arm
* armless - without arms
* armament - weapons and equipment
* armory - a place where weapons are stored
* armoured - protected with armor
* armpit - the hollow under the arm
* armstrong - strong in the arms
It's important to note that the prefix "arm-" is relatively uncommon compared to other prefixes like "un-", "re-", or "pre-".