bare shoulder; one shoulder exposed
Refers to a shoulder left uncovered, often in the context of clothing slipping off or being deliberately bared. Commonly appears in set phrases like 片肌脱ぐ (to lend a hand, literally 'to bare one shoulder').
One shoulder of her yukata was slipped off.
He pitched in and helped with the work (literally, bared one shoulder).
one shoulder exposed
A native compound of 片 “one side” and 肌 “skin,” denoting one exposed shoulder or one side of the upper body.