Person you invite to go somewhere when your preferred person cannot go. Can be male or female (non-gender specific).
"You can't make it? I better call my back-up bitch!"
The next person in line that you ask to do something with after your preferred person isn't able to.
guy 1: "My girlfriend can't make it for lunch with me today...do you want to go?"
guy 2: "What, am I your back-up bitch?"