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Related to donkey's years: donkey work
A long time. I haven't been here in donkey's years—I can't believe how much the town has changed.
A long time, as in I haven't seen her in donkey's years. This expression punningly alludes to the considerable length of the animal's ears. [Early 1900s]
donkey's yearsBRITISH, INFORMAL
If something lasts or has been happening for donkey's years, it lasts or has been happening for a very long time. I've been a vegetarian for donkey's years. He owns some old iron mines that haven't been used in donkey's years. Note: This expression was originally `as long as donkey's ears', which are very long. The change to `donkey's years' may have come about partly because the expression is used to talk about time, and partly because the original form is difficult to say clearly.