Booze that is carried in the purse in order to have one cheap drink at the beginning of a night out. Possibly contained in a flask, but most often an entire bottle of something picked up from the convenience store on the way out the door. Especially useful in foreign countries where drinking on the street is acceptable.
I got my purse booze, so I'm ready to party! or Yeah, grab me a beer, I'm going to head to the bathroom to finish up this purse booze.