donkey's years definition by Webster's New World
Webster's New World College Dictionary Copyright © 2010 by Wiley Publishing, Inc., Cleveland, Ohio. Used by arrangement with John Wiley & Sons, Inc.
Informal a very long time
donkey's years - Phrases/Idioms
A long time, as in I haven't seen her in donkey's years. This expression punningly alludes to the considerable length of the animal's ears. [Early 1900s]