Arm1  n. 1 upper limb of the human body from shoulder to hand. 2 forelimb or tentacle of an animal. 3 a sleeve of a garment. B arm support of a chair etc. C thing branching from a main stem (an arm of the sea). D control, means of reaching (arm of the law).  arm in arm with arms linked. At arm's length at a distance. With open arms cordially.  armful n. (pl. -s). [old english]