Resembling or characteristic of a donkey; stubborn, obstinate, or foolish like a donkey is stereotyped to be.
From 'donkey' plus the suffix '-ish' meaning 'resembling or characteristic of.' The negative associations come from medieval folk traditions that portrayed donkeys as stubborn and unintelligent, though this is unfair to actual donkeys.
Here's the thing: donkeys are actually incredibly intelligent and remember things for decades, but because they refuse to do dangerous things that would get them killed, people called them 'stubborn'—basically, donkeys have better survival instincts than we gave them credit for!
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