It may also stand for a white girl whose father is famous.
Most likely though, the main defintion would be an annoying white girl who really gets on your nerves
She's a miley.
He's a Miley.
We are Miley.
Girl 2: That is so Miley.
Girl: You're so Miley.
Miley Cyrus: Thank you!
Girl: Only you would take that as a compliment, you fucktard.