Designated Ugly Fat Friend. A book written by Kody Keplinger. Also a movie. Basically it's the person in a group of friends that is less popular than the rest, isn't as pretty/handsome, or they are a bit thicker than the rest. Sometimes it can be a freshman with a group of juniors or seniors. The Duff can be someone other people go to to get information on the Duff's hotter friends.
Emily = The Duff
Hot guy : Yo Emily!

Emily : *omg a hot guy is actually talking to me!!*
Hot guy : hey, do you know if your friend Hannah has a date to homecoming? I want to ask her to go with me.
by NWinchester January 12, 2016
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duffness- A name you can call someone or something without offending them up front. Clumsy, silly, jerk, funny, asshole, comedian, joker, dork, jerkoff, lame, pussy, fatass, drunk, idiot, rapper, skelator, bandit, clown, jester, tattoo, pig, fool, cholo, ganster, thug.
Kid tells his mom: Look at that clown his a duffness!
A boyfriend tells his girlfriend: Your a duffness for acting like that.
Audience: Look at the rapper acting like a duffness after the show.
The man was at the resturant eating a burger like a duffness
The duffness got cought robbing a bank
by T.F. King #1 May 11, 2011
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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.
by Guade April 2, 2015
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UK usage

1) Something that's flawed or faulty

2) To duff up, to assault, beat up etc
He sold me a duff DVD so I duffed him up.
by black flag May 31, 2004
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Get off your duff and help me with these dishes!
by Anonymous January 3, 2003
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A unattractive female who is always seen with another female who is marginally more attractive. It is usually impossible to score with the hotter female without the duff receiving her fair share of the action.
Damn! I so wanna tap that chick.. but goddamn, she always has that duff Nicole hanging off her. Look at that fat mess!
by Steve0 August 21, 2004
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marijuana that has already been run through a vaporizer at least once.
all i've got left to vaporize is some duff from last week, but we can still get pretty high.
by stoner June 18, 2006
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