John Doe

Books that documented and taught the legal profession in England were using the names John Smith, John Doe, Richard Roe, and others as generic placeholder names (for roles, such as plaintiff, defendant, petitioner, etc) by the mid-seventeenth century (perhaps earlier). Compare also Tommy Atkins. Though the rationale behind the choices of Doe and Roe is unknown, there are many suggested folk etymologies. Other fictitious names for a person involved in litigation in medieval English law were "John Noakes" (or "Nokes") and "John-a-Stiles" (or "John Stiles"). The Oxford English Dictionarystates that John Doe is "the name given to the fictitious lessee of the plaintiff, in the (now obsolete in the UK) mixed action of ejectment, the fictitious defendant being called Richard Roe".

来源：wiktionary