What is the origin of the idiom "on one's last legs" and when to use it?

The idiom "on one's last legs" is believed to have originated in the 17th century and was used to describe animals that were near death and could barely stand up. The phrase now means to be in a very weak or poor state, often due to age or illness. The phrase is often used to describe people or things that are nearing the end of their useful life.