Talk the hind leg/legs off a donkey
To talk volubly or excessively dates from the early 19th century but first appears in Cobbett’s Weekly in 1808, an American publication, as ‘talking a horse’s hind leg off’. The allusion is to the long legs of a horse and therefore the ability to talk for a very long time. It seems as if the British substituted ‘donkey’ for ‘horse’ sometime later during the 19th century. There are several other less known variations such as, talk the legs off ‘a chair, a dog or an iron pot’. Sometimes other animals and different body parts are substituted, such as ‘talk the hair off a monkey’ and even ‘talk the stripes off a zebra’ for an African flavour.