"to insist on payment of debt," 1620s, perhaps related to dunnen "to sound, resound, make a din," or shortened from dunkirk (c.1600) "privateer," a private vessel licensed to attack enemy ships during wartime, from Dunkirk, French port from which they sailed. Yet another, less likely, theory traces it to a Joe Dun, supposedly a London bailiff famous for catching defaulters. Related: Dunned; dunning.
O.E. dunn "dingy brown, dark-colored," perhaps from Celt. (cf. O.Ir. donn "dark"), from PIE *donnos, *dusnos "dark."