donkey's years

1876 or earlier. Possibly chosen because donkeys can be long-lived animals. Likely directly related to donkey's ears, a less frequently used rhyming slang term for years.https://wordhistories.net/2016/06/30/donkeys-years/

• "Didn't you ask him how long it had been in his possession? I can't recollect that. I might have done so. Didn't he say for years and years and donkey's years?"
• "With a heavy make-up, you’ll be the cutest vamp I’ve seen in donkey's years."