Triceps brachii muscle
The triceps is named because it has three distinct bundles of muscle that originate from three points - one on the scapula and two on the humerus. These bundles join near the insertion point at the elbow. It is innervated by the radial nerve.
The triceps is by far the largest muscle in the upper arm, usually accounting for 60-70% of the total muscle mass.
The long head originates from the infraglenoid tubercle of the scapula; the lateral head from the upper half of the posterior of the humerus. The medial head originates from the lower half of the posterior of the humerus.
In Latin, triceps means three-headed, referring to the three sections of the muscle. Brachii means arm. So the triceps brachii muscle is the three-headed arm muscle.