a long time: I haven't seen you in a coon's age!
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British Dictionary definitions for coon's age
US slang 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
Other Idioms and Phrases with coon's age
Also, a dog's age. A very long time, as in I haven't seen Sam in a coon's age, or It's been a dog's age since I went to the ballpark. The first phrase rests on the mistaken idea that raccoons (“coons”) live a long time. The variant may reflect a similar assumption but the true origin is not known. [c. 1835] Also see donkey's years.
The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.