Source : Webster's Revised Unabridged Dictionary (1913)

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.

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}]

dunning
     See {dun}