What is the origin of the idiom "one foot in the grave" and when to use it?

The idiom "one foot in the grave" has uncertain origins, but it is commonly believed to have evolved from the imagery of a person who is so close to death that they have one foot already in the grave, indicating their imminent demise. The phrase has been used in this figurative sense for centuries to describe the elderly or those in poor health who are nearing the end of their lives.