What does "John Doe" mean?

Also, John Q. Public; Joe Blow; Joe Doakes; Joe Zilch. An average undistinguished man; also, the average citizen. For example, This television show is just right for a John Doe, or It's up to John Q. Public to go to the polls and vote. Originally used from the 13th century on legal documents as an alias to protect a witness, John Doe acquired the sense of “ordinary person” in the 1800s. The variants date from the 1900s. Also see Joe six-pack.

Also, Jane Doe. An unknown individual, as in The police found a John Doe lying on the street last night, or The judge issued a warrant for the arrest of the perpetrators, Jane Doe no. 1 and Jane Doe no. 2. [Second half of 1900s]