Numerius Negidius
Numerius Negidius is a name used in jurisprudence in ancient Rome based on a play on words: Numerius Negidius means "one who denies (negat) that he should pay (numerare)", and was used specifically to refer to the defendant in a hypothetical lawsuit.
The plaintiff was referred to as Aulus Agerius. Aulus is a legitimate, if rare, Roman praenomen, and Agerius suggests the Latin verb ago, meaning "to put in motion", as it is the plaintiff who sets a lawsuit in motion.
One well-known legal formula, a model instruction to the judge in a civil lawsuit, began as follows: Si paret Numerium Negidium Aulo Agerio sestertium decem milia dare oportere, meaning "If it appears that Numerius Negidius ought to pay Aulus Agerius ten thousand sesterces". In actual use, the names and amounts would be changed to the appropriate values.
The initials N.N. can also stand for "name unknown" (nomen nescio), a placeholder name roughly equivalent to John or Jane Doe, Thomas Atkins, etc.