A person who you had a close and meaningful relationship with and who was a
big part of your life, but no longer is, usually due to a Falling Out or breakup. This person is typically an ex girlfriend, ex boyfriend, good friend, or family member that defined your past life, but seems to have vanished from your current life completely, whether you like it or not. In many cases, one
will pretend that this person no longer exists, or never existed in the first place, due to negative past memories of said person.
This is synonymous with a "somebody that I used to know", from the
2012 Gotye song, and comes from the newspeak word "unperson" in the George Orwell novel
1984, meaning a person who has been seemingly erased from
existence.