1960s: origin unknown; perhaps related to donkey's years (see donkey).

• donkey from late 18th century:

  Before the late 18th century a donkey was an ass. At first the word donkey was used only in slang and dialect, and its origin is lost. Early references indicate that it rhymed with monkey, and this has prompted some to suggest that it comes from the colour dun (Old English) or from the man's name Duncan. The expression for donkey's years, ‘for a very long time’, is a pun referring to the length of a donkey's ears and playing on an old pronunciation of ears which was the same as that of years. The British expression yonks, with the same meaning, may derive from it. See also easel