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30.
n. an inert substance used to dilute a narcotic such as cocaine.

The substance must be similar in appearance to the narcotic so as to satisfy the suspicions of the narcotic purchaser. The introduction of 'duff' to a narcotic increases the profit margin for the seller if the selling price of the product is not adjusted for the amount of duff introduced.

See also cut
"This shit isn't anything but duff!" A phrase one may use when the narcotic purchased for one's pleasure does not administer the desired effect.
by Oreck December 24, 2005
 
1.
Designated Ugly Fat Friend
Two hot chicks at a bar will have a really nasty fat ugly bitch hanging out with them, refered to as a "DUFF".

"Wow that fat DUFF is hanging out with those two attractive ladies, too bad."
by Cobra January 26, 2003
 
2.
the type of beer Homer Simpson drinks
pass me another Duff Mo
by :o) April 25, 2003
 
3.
Designated Ugly Fat Friend.
That 3rd girl is definitely the duff of the group.
by G January 02, 2004
 
4.
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 20, 2004
 
5.
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
 
6.
An abbreviation for 'Designated Ugly Fat Friend'. Often in many large groups of women (usually adolecents) there will be a D.U.F.F. as a means to look better by comparison.
I only hang out with her becuase I need a D.U.F.F., otherwise, she'd be, so, history.
by B.a.m December 20, 2005
 
7.
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