'Xià bā' literally combines 'xià' (down, lower) and 'bā' (a morpheme with no independent meaning in this context, historically related to 'mouth' or 'jaw' in classical usage). Together, they refer specifically to the chin—the fleshy, protruding part of the lower face below the mouth and above the neck. It is an anatomical term used in everyday speech, medical contexts, and descriptions of facial features or expressions.

This word is commonly used when discussing appearance ('a strong chin'), health ('swollen chin'), hygiene ('shaving the chin'), or gestures ('resting one's chin on the hand'). Unlike English 'jaw', which may refer to the entire bone structure including teeth and chewing muscles, 'xià bā' focuses narrowly on the external, visible lower facial contour. It’s neutral in register and appears in both spoken and written Chinese, though rarely in formal scientific writing where terms like '下颌 (xià hé)'—referring to the mandible bone—are preferred.