donkey: [18] The usual English word for ‘donkey’ from Anglo-Saxon times was ass, and donkey is not recorded until Francis Grose entered it in his Dictionary of the vulgar tonge 1785; ‘Donkey or Donkey Dick, a he or Jackass’. No one really knows where it came from. The usual explanation offered is that it was based on dun ‘brownish grey’ and the diminutive suffix -ey, with the intermediate k added in imitation of monkey (donkey originally rhymed with monkey). => dun[donkey etymology, donkey origin, 英语词源]