The character 下巴 is formed as a straightforward pictographic and ideographic combination: 下 (xià) means "below" or "under", and 巴 (bā), in its ancient pictographic form, is thought to depict a person with a prominent jaw or chin. When combined, they create a direct visual-concept, the "part that is below" (下) specifically referring to the area beneath the mouth, which is the chin or jaw (巴), effectively forming a literal "lower jaw" or "chin".