Someone who attends or has attended a prep school, and is usually wealthy and Caucasian. Examples of preppy fashion include tying one's sweater around one's neck, duck boots, wearing polo shirts with the collar up, and anything pearl or pink and green. "Wannabe prep" refers to people who are snobby and not truly preppy. They might be found wearing Abercrombie and Fitch, for example, instead of Lacoste.
She goes to that exclusive Connecticut boarding school, and she's so preppy, she even wears her collar up.