The definition of a mutt is a dog that does not have a pedigree.(noun)
A mixed breed dog you get at the pound is an example of a mutt.
See mutt in Webster's New World College Dictionary
Origin: prob. contr. < muttonhead
See mutt in American Heritage Dictionary 4
Origin: Short for muttonhead. Word History: Clipping not of sheep but of a word having to do with sheep has given us our term mutt for a mongrel dog. Clipping or abbreviating words, a standard process of word formation, sheared mutt from muttonhead, a pejorative term meaning “a stupid person” based on the notion that sheep are stupid. Mutt in its first recorded use in 1901 is used in the same senses as muttonhead, but it is soon recorded (1904) as a term of contempt for a horse and then (1906) for a dog. We can be reasonably certain that the New Yorker critic writing in 1970 that “The cast includes a Sheepdog, . . . a Mutt Bitch,” had no awareness that a sheepdog would make the ideal mutt.
Learn more about mutt