A sturdy animal related to the horse, with long ears and a loud braying call, often used as a working animal. In informal speech, it can also be used as an insult for someone considered foolish or stubborn.
The origin is uncertain; it appears in English in the late 18th century. It may be a playful alteration of 'dun' (a dull gray-brown color) plus the '-key' ending, or influenced by the pet-name pattern like 'monkey'. Before 'donkey', English speakers usually said 'ass'.
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