A girl who flirts with many, many guys at once, leading them on and ultimately screwing them each over. Flirt Sluts are generally immature with relationships, and/or have self-esteem issues.
If you know a girl like this, stay the fuck away from her. Trust me, they aren't worth the trouble, and shes not good for your sanity.
1: Hey! Did you hear that Abby is flirting with Ryan AND Cameron now?
2: What the fuck, she's flirting with me as well.
1: Damn man, she must be a "Flirt Slut". Get away now, you don't want to stay around her. She's a bitch.
2: Damn right she is...whore.