Where did John Doe come from?

Factual Questions
1

Where did the practice of naming an unidentified person John Doe come from? What makes that name so all-encompassing? Was it just a random choosing or is there some reason behind it?

2

Cecil Adams on Where does the name “John Doe” come from?

3

Trying to become anonymous? It won’t work, Boo-boo.
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4

Thanks, Manhattan, I couldn’t find that before.

5

He’s from Anytown, USA. Of course.
Peace,
mangeorge