for those of us who live outside the city, a city girl is a girl who lives in the city who you hook up with on business or recreational trips to the city. This act is usually done behind the back of a significant other or friends.
I've got a conference in Chicago next Thursday. Will be a nice change though I haven't seen my city girl in weeks.
Bro #1: You taking your girlfriend with you to Chicago?
Bro #2: No way bro, gonna meet up with my city girl.