The Mirriam-Webster dictionary defines "unperson" with the following;

An individual who usually for political or ideological reasons is removed completely from recognition or consideration

There is no etymology of the word on that site.

I appears in George Orwell's novel "Nineteen Eighty-Four" which was publish in 1949. You can read this for free the George Orwell web site.

Elsewhere, the word "nonperson" (also attributed to Orwell) is defined as:

Someone whose existence or presence is not recognized.

Someone whose existence is denied or ignored by a government, political party, or the like, often as a punishment for disloyalty or dissent and sometimes resulting in the loss of personal liberty; unperson.