The Collaborative International Dictionary of English v.0.48:

Dun \Dun\ (d[u^]n), v. t. & i. [imp. & p. p. Dunned (d[u^]nd);
   p. pr. & vb. n. Dunning (d[u^]n"n[i^]ng).] [AS. dyne noise,
   dynian to make a noise, or fr. Icel. dynr, duna, noise,
   thunder, duna to thunder; the same word as E. din. [root]74.
   See Din.]
   To ask or beset (e.g., a debtor), for payment; to urge
   importunately.
   [1913 Webster]

         Hath she sent so soon to dun?            --Swift.
   [1913 Webster]