a girl in a group of other girls that is drastically uglier and broader than the rest often making them look better(may also be a cockblock). the term is derrived from the phrase "Designated ugly fat friend". The -y at the end is added so the word can be used freely in the presence of the duffy without making it obvious that you are making fun of her.
Yo those girls over there are pretty hot...aw man dude, they got a duffy. I call she's not mine.