noun

• (dialectal) Dough.

noun

• (Scotland, Northwestern US, British Columbia) Decaying vegetable matter on the forest floor.
  "Forest fires have often been started from wantonness; for the sake of making a big blaze, fires will be carelessly left by guides, or will be smouldering in the duff,* where it will burn for weeks. I have seen the smoke from fires in the duff even after the snow has fallen. * Local term for the vegetable growth covering the forest ground of the Adirondacks. under the spruce trees, the falling needles accumulate to considerable depth, forming the "spruce duff," a peculiar and interesting variety of forest humus."
• (British) A mixture of coal and rock.
• (slang) The bits left in the bottom of the bag after the booty has been consumed, like crumbs.
• (baseball, slang) An error.

adj

• (UK) Worthless; not working properly, defective.
  "Why do I always get a shopping trolley with duff wheels?"

noun

• (US, slang) The buttocks.