Talk the hind legs off a donkey

The image is vivid, someone talking so relentlessly they could supposedly persuade an animal to give up its own legs, but no one has ever pinned down exactly where or when it was first said, or why a donkey specifically rather than any other animal. The phrase is well attested in English from the 19th century onward, always used in more or less its current sense, with the exaggeration doing all the work and no documented backstory behind it.