A flat slut is somebody named Amy who has been in a terrible sexual accident and had their face smashed against a woman's extremely flat breasts. This caused her face to become as flat as a piece of paper. If you ever meet a Flat Slut, don't get too close because she will reel you in and turn you into a condescending dick who gets really annoyed when their threesome with Amy doesn't go to plan because Geraldine doesn't know how to use a vaginal condom.