二头 = 'two-headed' - the biceps literally has two heads (long head + short head) where it attaches to the shoulder. 三头肌 (triceps) follows the same logic with three heads. Anatomically transparent.
usage
Common gym vocab pairs: 二头肌 (biceps), 三头肌 (triceps), 胸肌 (pecs), 腹肌 (abs), 背肌 (back). 肌 is the suffix for muscle group.
