Where does the term "work out" come from?

Maybe they used to lift things outside to build muscle. Just one example, rocks. I’m obviously just guessing, though.

According to the OED, it came from the world of boxing, where it originally referred to a practice bout. Here’s the first recorded use (1909 R. A. WASON Happy Hawkins 161): “I expect to give it a fair good work-out before I’m through with it.”

I take it as “I’m going to the gym to work out all this stress!”