duff (Adjective) — (informal) no good, useless, broken.
duff (Noun) — A stiff flour pudding steamed or boiled usually and containing e.g. currants and raisins and citron.
duff (Noun) — (informal) buttocks.
duff (Verb) — (informal) hit, beat up.
duff (Verb) — (in golf) mishit the ball.