Mutt can now be used with either affection or disdain to refer to a dog that is not purebred, but in the word's early history, in the U.S. around the turn of the 20th century, it could also be used to describe a person—and not kindly: mutt was another word for "fool." The word's history lies in another insult. It comes from muttonhead, another Americanism that also means essentially "fool." Muttonhead had been around since the early 19th century but it was not unlike an older insult with the same meaning: people had been calling one another "sheep's heads" since the mid-16th century.