expression, godan verb (-u)
be inseparable; live in perfect harmony
Literary expression, often used for a married couple who are always together and deeply united. The image is of a form and its shadow accompanying each other.
あの老夫婦はまさに形影相伴う仲だ。
That elderly couple truly lives in perfect harmony, always together.
Also means being inseparable or united, but 一心同体 emphasizes being of one mind and body, while 形影相伴う focuses on the image of always accompanying each other like a form and its shadow.
Derived from classical Chinese, combining 形 (form), 影 (shadow), 相 (mutually), and 伴う (accompany). The exact historical derivation is uncertain; the expression is conventionally associated with the inseparable bond of a married couple.