A woman who is considered very attractive, only because those around her are pretty average. If she was in another setting, she wouldn't be much to look at, at all.
An expression used by insult comic Lisa Lampanelli.
Barry: "I picked this chick up in the bar last night and took her home."
Mike: "Was she hot?"
Barry: "Nah, she was a Kansas 10."
Mike: "So, about a New York 3?"