The idiom "形影不离" (xíng yǐng bù lí) is formed through a vivid metaphorical juxtaposition: "形" means physical body or form, "影" means shadow, and "不离" means never to leave or inseparable. Literally, it describes a person and their shadow being incapable of separation, a poetic image used to convey an extreme state of closeness and constant companionship between two entities, such as close friends, lovers, or inseparable things. The phrase leverages the inherent, unavoidable connection between an object and its shadow to figuratively express an unbreakable bond, illustrating a classic Chinese linguistic pattern where a concrete natural phenomenon is employed to abstractly describe human relationships or states of being.