Merriam-Webster Collegiate Dictionary - dun

 I. adjective  Etymology: Middle English, from Old English ~n — more at dusk  Date: before 12th century  1.  a. having the color ~  b. of a horse having a grayish-yellow coat with black mane and tail  2. marked by dullness and drabness  • ~ness noun  II. noun  Date: 14th century  1. a ~ horse  2. a variable color averaging a nearly neutral slightly brownish dark gray  3. a subadult mayfly; also an artificial fly tied to imitate such an insect  III. transitive verb  (~ned; ~ning)  Etymology: origin unknown  Date: circa 1626  1. to make persistent demands upon for payment  2. plague, pester  IV. noun  Date: 1628  1. a person who ~s  2. an urgent request; especially a demand for payment