MMFU Posted in M Best definition MMFU My Middle Finger Up.-Created by Marcel Kim Supporters, Jean Luc rad. Donald Kim. MMFU to the 15 year old boys who look like they take roids but in reality only photo shopped their pics. How satisfied are you with the definition? Looking for MMFU meaning? Delighted that you discovered the greatest determine on our web site! Recently this definition was actually observed 118 times.