In the discussions on this idiom that I have read, I have never seen a definitive answer. One theory is that it is two idioms coming together, and another suggests that it means that the person could persuade the donkey to sit down, the unlikeliness of this being the testament to their powers of persuasion (
http://www.phrases.org.uk/bulletin_b...ages/710.html). I am afraid that the truth may have got lost in the mists of time.
talk hind legs off a donkey 
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