The Collaborative International Dictionary of English v.0.48:

Dun \Dun\, a. [AS. dunn, of Celtic origin; cf. W. dwn, Ir. &
   Gael. donn.]
   Of a dark color; of a color partaking of a brown and black;
   of a dull brown color; swarthy.
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         Summer's dun cloud comes thundering up.  -- Pierpont.
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         Chill and dun
         Falls on the moor the brief November day. --Keble.
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   Dun crow (Zool.), the hooded crow; -- so called from its
      color; -- also called hoody, and hoddy.

   Dun diver (Zool.), the goosander or merganser.
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