What is the origin of the idiom "bag of bones" and when to use it?

The exact origin of the idiom is unclear, but it likely developed as a vivid and straightforward way to depict extreme thinness. It is used metaphorically to describe someone who is excessively thin or lacking body mass. It highlights the person's skeletal appearance and suggests a lack of flesh or muscle. This phrase is typically used in informal conversations or when discussing someone's physical appearance in a descriptive manner.