I Just Learned Why We Say 'Donkey's Years', And I'm Shocked

There are some things, like WiFi and satellites, that I trust for no reason ― I have no idea how they really work. 

The same goes for certain words and phrases (what is a “fangled” when it’s not “new”?). 

We’ve written before at HuffPost UK about how the “deserts” in “just deserts” is said like the sweet treat, pronounced like the sandy area, and means neither.

But what about “donkey’s years” to mean a long period of time? Are donkey years exceptionally long? I thought they had a longish, but not wild, lifespan.