Definitions by Guade

Duff 

Designated,ugly,fat friend.

You don't necessarily have to be fat,or ugly to be one. the duff is considered a duff when he/shes significantly less attractive out of the group of friends they hang out with and both boys and girls can be one.
They are often used to get information about his/her friends.

1# "Damn, those girls are hot...well except for that one"
2#"yeah..maybe we can get some info on them from her?"
also shown in the movie"The Duff" if you need a visual example.

Duff by Guade April 2, 2015