What is the origin of the idiom "a dog's life" and when to use it?

The exact origin of the idiom "a dog's life" in the sense of describing a life of hardship or difficulty is not definitively documented. It is a long-standing expression that likely stems from the perception of dogs facing difficult or challenging conditions in the past, possibly when they were used for laborious tasks or faced harsh living conditions. While the exact historical origin is unclear, the phrase has been in use for an extended period.