What is the origin of the idiom "a fine line" and when to use it?

The idiom "a fine line" originated from the literal concept of a line, which is a narrow and distinct mark or boundary. In a figurative sense, the phrase refers to a subtle or delicate distinction between two similar or closely related things. It is used to highlight the idea that there is a small margin or difference that separates two concepts, actions, or outcomes, often implying that the boundary can easily be crossed or misunderstood.