Definition - a dull-witted person
Muttonhead is not quite so old as many of the words in this list, in use only since the early 19th century. It makes up for this lack of lineage with a degree of oddity that sets it apart from the older words. Muttonhead is a combination, obviously enough, of head, and mutton (“the flesh of a mature sheep used for food”). It also is the word from which we get mutt, a word which has been applied, since the end of the 19th century, to horses (of the run-down variety), people (of insignificance), and dogs (of indeterminate or mixed breed).
To recap: a word for people, which was based on sheep, shortened and became a term for horses (or people), and now is most commonly applied to dogs.
Mankind are bored enough with scholastic mutton heads on commencement day; but to have the wretches continually grinding on their patience from the columns of a newspaper is past endurance.
— National Advocate (New York, NY), 13 Jul. 1826