Definitions of dun:

• noun:   horse of a dull brownish gray color
• noun:   a color varying around light grayish brown

  Example: "She wore a dun raincoat"

• verb:   make a dun color
• verb:   cure by salting

  Example: "Dun codfish"

• verb:   persistently ask for overdue payment

  Example: "The grocer dunned his customers every day by telephone"

• verb:   treat cruelly
• adjective:   of a dull grayish brown to brownish gray color

  Example: "The dun and dreary prairie"

• name:  A surname (very rare: popularity rank in the U.S.: #43968)