Webster's New World College Dictionary Copyright © 2010 by Wiley Publishing, Inc., Cleveland, Ohio.
Informal a very long time
Used by arrangement with John Wiley & Sons, Inc.
donkey's years - Phrases/Idioms
The American Heritage® Dictionary of Idioms by Christine Ammer. Copyright © 2003, 1997 by The Christine Ammer 1992 Trust. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
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]