The terms Ploni and Almoni are used in Hebrew as a placeholder, particularly in legal jargon, and also to refer to any undistinguished someone or something.
This practice is also prevalent throughout the world, with appropriate terminology to suit each language and country. For example, in the United States the term "John Doe" or "John Smith" is used, or "Jane Doe" for a female. These same terms are also used in other major English-speaking countries, in addition to "Tom, Dick and Harry" and "John Citizen."
In Arabic, "Fulan al-Fulani" is used; in French you may hear "Jean Dupont"; while in Yiddish there are those that say "Yankel Tuches." For every language, there is a corresponding term for this placeholder, but the origin of the concept of ploni almoni actually dates back to the Bible, where the name of an unspecified place or person is referred to in this manner - in certain cases deliberately, and in others merely because the name is unknown.
We first encounter the term Ploni Almoni in I Samuel (21:3), when David is discussing a business deal with Abimelech and he refers to the location where their negotiations will take place as Ploni Almoni. The commentaries explain that since David wanted to keep the location undisclosed, he intentionally alludes to it in this manner, thereby preserving its anonymity.
Their explanation of the term is based on the derivation of the words - Ploni from plai, which can be translated as "wonder," and Almoni from ilam, someone who is mute.
The term is used again in II Kings (6:8), when the king of Aram (Syria) is plotting war against Israel, and he also refers to the place where his camp will be situated as Ploni Almoni, conveying his desire to keep the location confidential.
The first use of the term in the Bible to refer to an unknown person is in Ruth (4:1), when Boaz fails to remember the name of a passerby, seemingly a relative of his, and so to avoid embarrassment, he simply labels him Ploni Almoni.
The term is again used a few times in Chronicles and Daniel. In the time of the Talmud, the term evolved as the established manner in which to refer to a plaintiff in any legal discussion. This practice has continued to this day.