Source : Webster's Revised Unabridged Dictionary (1913)

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.

         Summer's dun cloud comes thundering up.  -- Pierpont.

         Chill and dun Falls on the moor the brief November day.
                                                  --Keble.

   {Dun crow} (Zo["o]l.), the hooded crow; -- so called from its
      color; -- also called {hoody}, and {hoddy}.

   {Dun diver} (Zo["o]l.), the goosander or merganser.

Dun \Dun\, v. t.
   To cure, as codfish, in a particular manner, by laying them,
   after salting, in a pile in a dark place, covered with salt
   grass or some like substance.

Dun \Dun\, n. [See {Dune}.]
   A mound or small hill.

Dun \Dun\, v. t. & i. [imp. & p. p. {Dunned}; p. pr. & vb. n.
   {Dunning}.] [AS. dyne noise, dynian to make a noise, or fr.
   Icel. dynr, duna, noise, thunder, duna to thunder; the same
   word as E. din. ???. See {Din}.]
   To ask or beset, as a debtor, for payment; to urge
   importunately.

         Hath she sent so soon to dun?            --Swift.

Dun \Dun\, n.
   1. One who duns; a dunner.

            To be pulled by the sleeve by some rascally dun.
                                                  --Arbuthnot.

   2. An urgent request or demand of payment; as, he sent his
      debtor a dun.

Source : WordNet®

dun
     adj : of a dull grayish brown to brownish gray color; "the dun and
           dreary prairie"
     [also: {dunning}, {dunned}, {dunnest}, {dunner}]

dun
     v 1: treat cruelly; "The children tormented the stuttering
          teacher" [syn: {torment}, {rag}, {bedevil}, {crucify}, {frustrate}]
     2: persistently ask for overdue payment; "The grocer dunned his
        customers every day by telephone"
     3: cure by salting; "dun codfish"
     4: make a dun color
     [also: {dunning}, {dunned}, {dunnest}, {dunner}]

dun
     n 1: horse of a dull brownish gray color
     2: a color varying around light grayish brown; "she wore a dun
        raincoat" [syn: {grayish brown}, {greyish brown}, {fawn}]
     [also: {dunning}, {dunned}, {dunnest}, {dunner}]