Donkey’s years

Curiously enough, this expression started life at the beginning of the 20th century as rhyming slang, donkey’s ears/years. By the 1920s, however, it had become donkey’s years, instead of donkey’s ears, meaning a very long time, with the rhyming aspect slipping away. Donkeys do in fact live for quite a long time compared with other farm animals, some of them, well into their sixties.