Designated Ugly Fat Friend
Doesn't necessarily have to be fat, or ugly, but is less attractive than the friends she/he hangs out with. Men/woman usually befriend a duff to get closer to his/her friends. A duff mainly gets used for info and or help, and is often lead on mistakenly or purposely. Do not allow the duff to realize who he/she is and why you are using them, because you will 100% lose them as a friend. If you even care.
Guy 1: "Hey see those hot chicks? Except that one, she's so the duff."
Guy 2: "Yah man? Let's try to be her friend, maybe get some info on her hot friends"
Guy 1: "let's do it!"