It's a girl who comes from a more affluent, more educated part of a city, or belongs to an upper social class of people. The noun "uptown" does not always convey this meaning; it can sometimes just refer to the northern part of a city. But as an adjective, uptown is used to convey a sense of upper social class. It can refer to an object or to a person. "That new dress gives you a very uptown appearance!" (You appear to be rich when you wear that dress!) "The new bus station is located uptown." (Location only) EDIT: You had it covered, fdmaxey! Didn't see your answer before I posted.

It refers to a girl who comes from a family with a fair amount of money, or acts like it, and may look down on working-class people as beneath her. It is not so much a place as an attitude. Just 'uptown' refers to the northern part of town, but it means something different when applied to a person. We can talk about someone who thinks they are uptown, or acts 'uptown'.