A person (usually a female) who flirts with multiple people on a daily basis, but not usually for any serious intention of going past the flirting stage (e.g. dating, relationship, etc). Some dating may occur from a flirt bag, but it's usually short-lived due to the natural habits of a flirt bag. A common place to find this flirt bag behavior would be at a workplace with a medium to large amount of employees.
Employee 1: Damn... each day that girl is flirting with a different worker every time I see her. It's all she does!
Employee 2: Yep, she's a flirt bag.