a duff is another definition of a noob; a term used when sombody makes a stupid mistake or does something stupid.
Alex: oh my god my ball hairs itch like hell, man.
Morgan: wait, you have hair on your balls.
Alex: wow, your a fucking duff.
Morgan: wait, you have hair on your balls.
Alex: wow, your a fucking duff.
by DJCVans July 01, 2006