What is the origin of the idiom "skin and bone" and when to use it?

The phrase "skin and bone" is an idiomatic expression that originated as a descriptive comparison. It emphasizes the thinness of a person by comparing them to the bare minimum of flesh, consisting only of skin and bone. It is used to describe someone who is extremely thin, to the point where their skeletal structure is prominent and visible. It suggests that the person lacks significant muscle mass and body weight.