Are You The DUFF.... Designated Ugly Fat Friend

Designated - to mark or point out; indicate; show; specify.

The DUFF (Movie)

Frumpy high-school senior Bianca (Mae Whitman) has a rude awakening when she learns that her classmates secretly know her as the DUFF -- designated ugly fat friend -- to her prettier and more popular pals. Desperate to reinvent herself, Bianca enlists the aid of Wesley (Robbie Amell), a charming joc… Morek. In order to save her senior year from becoming a complete disaster, Bianca must find the confidence to overthrow a judgmental student (Bella Thorne) and revolutionize the school's social order.

Designated Ugly Fat Friend (Urban Dictionary)

Two hot chicks at a bar will have a really nasty fat ugly friend hanging out with them, refered to as a "DUFF".

"Wow that fat DUFF is hanging out with those two attractive ladies, too bad."