Definition & Bedeutung von "gym rat"

What is the origin of the idiom "gym rat" and when to use it?

The idiom "gym rat" is a relatively recent expression and does not have a well-documented historical origin. However, it likely derives from the concept of a rat being a creature that frequently inhabits or spends a significant amount of time in a particular place. It is used to refer to an individual who spends an excessive amount of time at the gym, often to exercise and work on their fitness.