John/Jack is perhaps one of the most common names in the Western world, and Jane is it's female equivalent.
Doe or D.O.E. is/was an acronym for Dead On Entry (now more commonly D.O.A. - Dead On Arrival).
If a person's identity was unknown, this was the only thing written on the body's ID tag, attached to the big toe.
If you watch the crime programmes you will notice that the mortician might say "We have a John/Jane Doe", meanining we have a body, identity unknown, found dead on entry into hospital.