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
1785, originally slang, perhaps a diminutive from dun "dull gray-brown," the form perhaps influenced by monkey. Or possibly from a familiar form of Duncan (compare dobbin). The older English word was ass (n.1).
