McFlatAss Posted in M Best definition McFlatAss Someone who has no butt but is also very short. He/she talks like a country girl/guy and will say there thick but they arenât. OMG, she is such a McFlatAss I canât believe her she talks like a country bitch too. How satisfied are you with the definition? Looking for McFlatAss meaning? Delighted that you discovered the greatest determine on our web site! Recently this definition was actually observed 113 times.