"he can talk the hind legs off a donkey!"
definition: A person who is excessively or extremely talkative can talk the hind legs off a donkey
origin: During the history of this expression numerous other animals have featured in it: a horse, a dog, a cow and a bird (which of course has no hind legs). It was originally an expression of admiration for a person's powers of successful persuasion - a suggestion that one could bring about the impossible by talking. Nowadays, though said of a person admiringly, it is more usually a complaint.