Its the same concept as a cock blockbut its a uni-sex word, meaning it can be used for girls and guys. Its a person that basically prevents a person from hooking up with another.
A group of three or more girls and a duff come to a party. The duff is named Theresa Marie. All night Theresa Marie will not stop talking to the girls and prevents them from talking to the guys. She is constantly taking them away from the party and is being a total crotch blocker to the guys
A Designated Unattractive Fat Friend, or a Dumb Ugly Fat Fuck- a duff is a girl who is the ugly chick at the party that would be a considerable hook up with a high amount of alcohol ingested. A duff^2 is the girl at the party who is so fugly that no substance in any form can justify a random hook up
Hey chris, michelle just showed up, but she brought 3 fucking duffs, jill, heather, and theresa.
-one bottle of tequila later
Tom - yo bro, heather is bangin
Chris - yea but shes occupied with mike, how does theresa look.
Tom - Fuck that bro, i wouldnt go near her with a ten foot pole, she is clearly a Duff Squared