noun 1. Acronym for “designated ugly fat friend.” 2. A hit Y.A. book by Kody Keplinger about what happens when you’re in high school and you find out you’ve been cast as the less-attractive foil to your hot “friends.” 3. A 2015 movie adaptation of that book, starring ex-Disneyer Bella Thorne as an heiress to Mean Girls’ Regina George and Parenthood’s Mae Whitman as a DUFF bent on reinvention (and a little revenge).