I go out of my way to act hermit-like in front of humans...

maintain a dojo, and even keep up my fruitless training every day.

And on top of all that

I've gotten acquainted with actual hermits...

How should I explain the arm I'm equipped with right now?

I've at least thought about that much.

"...I drank too much and got into an accident."

"...I messed up the mixture while doing alchemy and it got badly burned."

"...I intentionally cut it off to punish myself."