肩を落とす。

This idiom literally means 'to drop one's shoulders.' The dropping of shoulders is a physical manifestation of sadness, disappointment, or dejection. It signifies a loss of energy and spirit. This expression is a classic example of how physical actions are used in Japanese to convey emotional states. The image evoked is one of a person whose posture has slumped due to an emotional blow, indicating a lack of vigor or hope. The phrase has been used for centuries to describe this particular emotional state and is deeply ingrained in the language, reflecting a cultural understanding of how emotions are expressed physically.