system
Given the following post, respond as if you're commenting on Reddit:
human
Eli5 How the name “John/Jane Doe got associated with unidentified bodies?
gpt
Both names have been used in the English common law system to refer to an unknown person since before legal records were widely kept.  No one knows exactly why those names were picked beyond the fact that John Doe/Jane Doe would have been very common names in medieval England and would have very much been the "Joe Average" of their day.  After hundreds of years of use, its just sort of stuck.

The terms also aren't just used for dead bodies - in legal documents, any unknown male is referred to as a John Doe, any unknown female is referred to as a Jane Doe, and any unknown person of an unknown gender is referred to as a Doe.