Mutt is an American expression for a mongrel dog. According to the dictionaries, the term mutt is thought to be a version of muttonhead, which enters English around the same time (beginning of the twentieth century) and means a fool or a dolt. There is no great mystery about this. Muttonhead is saying that something or someone resembles a sheep and mutt is used of people as well as dogs from the start. Of a person, this probably means they’re stupid. Of a dog, it probably just means that they have a shaggy and unkempt coat.
Daniel Cassidy, the Great Fraud, says that mutt doesn’t come from muttonhead. According to him, it comes from madadh or madra (mada in Dinneen) the Irish for dog (not mongrel.) These words begin with m but apart from that, they don’t sound much like mutt.
However, the funniest thing in this piece is the way that Cassidy tries to rubbish the derivation from muttonhead.
“Some Anglo-American lexicographers derive mutt from muttonhead, as in a sheep’s head. But a muttonhead is a dunderhead or a dolt. Most mutts are (street) smart.”
Well, that proves it then! Hearken to his cold, inexorable logic …
What a total putz!