noun

• (countable) A collector of debts, especially one who is insistent and demanding.
  ""The truth is, Mr. Curl, I cannot write when I am plagued about trifles; and a tiresome dun this morning put to flight every idea that I had in the world." "Mr. Maynard," said the bookseller, in a solemn tone, "it is very wrong to run in debt.""

verb

• (transitive) To ask or beset a debtor for payment.
  "And hath she sent so soon to dun?"
• (transitive) To harass by continually repeating e.g. a request.
  "Rich bitches who had to be dunned for their milk bills would pay him right now."

noun

• (countable) A newly hatched, immature mayfly; a mayfly subimago.
  "Also, duns are dull and generally sober colored, whilst spinners are more brightly colored and shining and their wings are clear and transparent."
• (countable, fishing) A fly made to resemble the mayfly subimago.
  "We have besides for this Month a little Dun call'd a whirling Dun (though it is not the whirling Dun indeed, which is one of the best Flies we have) and for this the dubbing must be of the bottom fur of a Squirrels tail and the wing of the grey feather of a Drake."

noun

• (archaeology) A structure in the Orkney or Shetland islands or in Scotland consisting of a roundhouse surrounded by a circular wall; a broch.
  "Smaller than the broch was the dun, another type of stone-built 'roundhouse'."