noun
arm
ARM/ɑːrm/
An upper limb extending from the shoulder.
His arm extends from the shoulder to the relaxed fingertips.

Etymology
Old English earm already named the upper limb. The English word preserves that basic bodily meaning, covering the limb from shoulder to hand.
In context
your hand is moving together with the arm from the shoulder. So it kind of gives you a bigger range of movements.
More examples
- The child wrapped both arms around the puppy.
- She works for the charity’s educational arm.
- The research arm of the company develops new materials.
Forms
arm · arms