close relationship; inseparable bond
Describes a very close, inseparable relationship, often between people or entities. The literal meaning is 'not side, not separation', implying constant togetherness. Somewhat formal or literary.
The two countries have an inseparable relationship.
A four-character compound (四字熟語) composed of 不 (not), 側 (side), 不 (not), 離 (separation). The literal meaning is 'not side, not separation', i.e., always together. The exact origin is uncertain, but it follows a common pattern of classical Chinese-derived expressions.