I'm at the doctors and we're trying to work out the last time I remember being weighed. “Must have been donkey's years then?” the doctor jokingly says as I prepare to be shocked and appalled by the reading on the scales. As I'm removing my clunky shoes, she pauses me in my tracks, asking where I think the phrase “donkey's years” comes from. Like her, I have no idea – it is simply a phrase, I too, have unthinkingly used in the past. We both guess it either has something to do with the duration of the average donkey's life or that like cats, donkeys have their own “years” (http://en.wikipedia.org/wiki/Cat_years) and leave it at that.
At home and slightly traumatised by my weigh-in, I decide to distract myself by finding the collectively agreed etymology of the phrase. Almost every link I click on unanimously agrees the phrase originally began as “donkey's ears”, rather than “years”, stemming from rhyming slang that alludes to the length of the animal's ears. It seems “donkey's ears” was shortened to “donkeys” as is characteristic of rhyming slang – consider “trouble and strife” (meaning “wife”) which is normally abbreviated to “trouble”. The rhyming of “ears” with “years” in “donkey's ears” is in keeping with the conventions of rhyming slang, whereas “years” instead of “ears” clearly isn't.
The phrase “donkey's ears” was apparently first recorded in 1916: "Now for my first bath for what the men call 'Donkey's ears', meaning years and years", while “donkey's years” is first documented in use several years later in the 1920s: "With a heavy make-up, you'll be the cutest vamp I've seen in donkey's years." The transition from “donkey's ears” to “donkey's years” most probably came from pronunciation and was also aided by the misguided belief donkeys have a particularly long life expectancy. The reality is working donkeys in poorer countries are likely to live between 12-15 years and those living the life of decadence in richer areas can have a lifespan of between 30 to 50 years. A Blackpool Pleasure Beach regular called Lively Laddie apparently made it until 62 and was a serious contender for the 'oldest living donkey' title (http://www.phrases.org.uk/meanings/donkeys-years.html).