While English uses phrases like John Doe or A N Other to refer to an unnamed or unspecified person, in Hebrew we say ploni almoni or just ploni. In discussing hypothetical scenarios, the Talmud uses ploni to refer to a witness, a bridegroom, or whatever the case maybe.
Ploni Almoni appears in Ruth, as the late Elimelech's relative who should marry Ruth but refuses. Rashi explains ploni to mean covered and hidden, deriving from a word looked at recently in this column - peleh (Ruth 4:1). Almoni comes from ilem, meaning silenced or muted. Ploni Almoni turned away from the historic act of embracing Ruth into the Jewish people. Therefore it is only fitting that his real name be blocked out.
In I Samuel David, in flight from Saul, does not wish to disclose his destination to the priest Ahimelech, so he says, "I have directed my men to the place of ploni almoni" (21:3).
In the Bible, ploni almoni is used when one wants to conceal information deliberately. In the Talmud and in modern Hebrew, ploni almoni is simply a useful expression to refer to no one in particular.
Today, ploni almoni is seeing a renaissance in cyberspace as way to sign off talkbacks anonymously. He even has his own facebook account.