dunyoudaoicibaDictYouDict[dun 词源字典]
dun: English has two words dun. The colour adjective, ‘greyish brown’ [OE], comes ultimately from Indo-European *donnos, *dusnos, which is also the source of English dusk. The now rather dated noun, ‘debtcollector’ [17], is an abbreviation of dunkirk, a 17th-century term for a ‘privateer’, a privately owned vessel officially allowed to attack enemy shipping during wartime.

It was originally applied from such privateers that sailed from the port of Dunkirk, on the northern coast of France, to attack British ships, and its connotations of unwarranted piracy soon spread metaphorically to one who was constantly importuning for the repayment of his loan.
=> donkey, dusk, obfuscate[dun etymology, dun origin, 英语词源]
dun (v.)youdaoicibaDictYouDict
"to insist on payment of debt," 1620s, perhaps related to dunnen "to sound, resound, make a din" (c. 1200, dialectal variant of 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. The oldest theory traces it to a Joe Dun, supposedly a London bailiff famous for catching defaulters. Related: Dunned; dunning. As a noun from 1620s.
dun (adj.)youdaoicibaDictYouDict
Old English dunn "dingy brown, dark-colored," perhaps from Celtic (compare Old Irish donn "dark;" Gaelic donn "brown, dark;" Welsh dwnn "brownish"), from PIE *donnos, *dusnos "dark."