First Name Unknown!
Many ages ago, when this ancient planet was not so ancient, long before man recorded his history, there was the time of Middle Earth, when man shared his days, with elves, dwarves, wizards, goblins, dragons and hobbits.
At the time, when introduced, Bilbo Baggins was required to say he is from Shire. Why? So that people would identify him, classify him. That nobody would ask him what Gollum had asked.
Bagginses? What is a Bagginses, my precious?
Maybe, for identification and classification of human races and geographical identity, a location to the name was required in ancient times. Gradually, this culminated into a required last name, that would identify caste, occupation or geographical identity.
Well in some countries in the modern world, it is not mandatory to have a last name. For instance, India. Every legal document can be created with just one Given Name. And this is my case.
However, in the USA it is mandatory for an individual to have a first and a last name, at least. You can not be allowed a legal entry into the USA if you do not have these two fields filled in your Immigration form. (I think, probably a database somewhere 50 years ago had a mandatory first name and a mandatory last name field. LOL!)
Ok then. If you have just one name in your country and you decide to come to USA, you need your name changed. The easiest and convenient of all is to just add a Last Name. However, if you can not or do not want to, you need to fill in every immigration/legal form with your legal name as the Last Name and FNU as first name. FNU stands for First Name Unknown.
It’s the 21st century! Long long away from Middle Earth. Look at our progress! Bravo!
Welcome to the United States, FNU :P