a very long time; eons.
Origin of donkey's years
1895–1900; probably orig. donkey('s) ears, as rhyming slang for years, with years replacing ears once rhyming orig. was forgotten
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2019
informal a long time
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012
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]
The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.