A coin, token, or medal, made by using two dies which were not originally intended for each other.
The term was first generally used in the latter part of the eighteenth century, and it may have been adopted from the " Token Collectors' Half-penny " of 1796, the re- verse of which represents an ass and a mule saluting each other, with the inscrip- tion, " Be assured, friend mule, you shall never want my protection."
The German name for this class of coins and medals is Zwittermunzen, and speci- mens exist dating from the early part of the sixteenth century.
See Also: Mule
Source: Frey's Dictionary (American Journal of Numismatics, Vol. 50, 1916)