To talk incessantly, either persuasively, or tiresomely. Someone who can talk the hind leg off a donkey definitely has the gift of the gab.
There are many other things that can have their legs talked off, including chairs, dogs, stoves, horses, and elephants, though I’ve never heard any of those used.
Possibly arising in Ireland around the early 1800s, the phrase refers to the fact that horses or donkeys rarely sit down on their behinds. So, to talk the hind legs off a donkey or horse is to talk so long that the animal becomes exhausted and collapses. The hind legs do not ‘fall off’ as in the related idioms ‘to talk someone’s ear off’ or ‘talk someone’s arm off’ but they ‘lose their legs’ as though they have fainted or collapsed.
The phrase ‘talking the hind leg off a horse’ was first recorded in 1808. Later ‘donkey’ became more popular. The quote where it appeared referred to the saying as an ‘old vulgar hyperbole’ (common exaggeration). Obviously, therefore, it had been around for quite some time prior to that.
Why are donkeys so apt in this expression? Because they are strong, sturdy, faithful and extraordinarily reliable animals that tolerate a great deal from us. So perhaps the phrase implies that the talker places a heavier load on the listener than anyone would put on a donkey. But ears are not known to collapse, so ‘donkey’ makes more sense.
Donkeys have been working with us for at least 4,000 years. Despite all this faithful service, we often don’t treat donkeys kindly in our language. ‘Donkey’ is a favorite insult for someone who is stupid or silly, and ‘donkey-work’ is hard, menial labor. But, since I like donkeys, I’ll point out that they are very sure-footed, which allows them to live and work in areas completely inaccessible to the much more respected horse.
In his song On Again! On Again! Jake Thackray writes something like “never mind donkeys, she can bore the balls off a buffalo.”
The poor donkey wouldn’t have a chance with that woman!