This idiom has my right knee written all over it. You'll see why.
It means to be capable of making decisions without advice. [Yep ... that's my knee.]
Use in Normal SpeechEdit
When discussing akward people/body parts.
"My right knee has a mind of its own. It started to hurt yesterday evening and I don't know why."
When expecting someone to make a decision without help.
"I thought you had a mind of your own, mate. Use it."