"The mule combines the strength of the horse with the endurance and surefootedness of the ass, and is extensively bred for certain employments for which it is more suited than either; it is ordinarily incapable of procreation. With no good grounds, the mule is a proverbial type of obstinacy." [OED]Properly, the offspring of a he-ass and a mare; that of a she-ass and a stallion is technically a hinny. Used allusively of hybrids and things of mixed nature. As a type of spinning machine, attested from 1797. Meaning "stubborn person" is from 1848; that of "narcotics smuggler or courier" first attested 1935. O.Fr. mul was replaced in Fr. by dim. form mulet, hence muleteer "mule driver" (1538), from Fr. muletier.
"For but ich haue bote of mi bale I am ded as dorenail" (c.1350).