Ye Olde Nincompoop

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

Definition - blockhead, ass

The English language is particularly rich in words which are capable of describing a person who makes questionable choices, and a large number of these words appear to end with -head. A very partial list includes bonehead, bufflehead, chowderhead, chucklehead, citternhead, dolthead, doughhead, dullhead, dumbhead, dunderhead, hammerhead, hardhead, jolter-head, leatherhead, loggerhead, muddlehead, noodlehead, pumpkin head, ramhead, squarehead, thickhead, and woodenhead. Of these perhaps none has such trenchant weight as the humble asshead.

The scholler went home with a heavy heart, bearing a letter to Master Scogin, how his scholler could not answer to this question, Isaac had two sonnes, Esau & Iacob, who was Iacobs Father. Scogin said to his scholler, thou foole and asse-head, doest thou not know Tom Miller of Osney?
— Andrew Boorde, The first and best part of Scoggins iests full of witty mirth and pelasant shifts, done by him in France, and other places: being a preseruatiue against melancholy, 1626

Definition - a dull and stupid person

As noted above, many terms of abuse in English are formed by adding -head to an existing word. When the language got tired of using the word head it began using synonyms, such as pate. Many such words are now obscure, although a few, such as addlepated are still in occasional use. Clodpate is one of the more obscure ones.

Come you Clod-pate, you Black-moore, what sayst thou to me? I fall upon the Peripatetics (you say) as superficiall Philosophasters.
— Thomas Vaughan, The man-mouse taken in a trap, 1650

Definition - blockhead

Doddypoll is an alteration of the Middle English dotypolle, a result of combining a word meaning “fool” (dote) with a word meaning … you guessed it, “head” (poll). The word has found use in the works of numerous writers since the 15th century, perhaps never so memorably as when Thomas Urquhart used it in his late 17th century translation of Rabelais.

The Cake-bakers were in nothing inclinable to their Request; but (which was worse) did injure them most outragiously, calling them pratling Gablers, lickorous Gluttons, freckled Bittors, mangy Rascals, shite-abed Scoundrels, drunken Roysters, sly Knaves, drowsie Loiterers, slapsauce Fellows, slabberdegullion Druggels, lubbardly Louts, cousining Foxes, ruffian Rogues, paultry Customers, sycophant Varlets, drawlatch Hoydons, flouting Milk sops, jeering Companions, staring Clowns, forlorn Snakes, ninny Lobcocks, scurvy Sneaksbies, fondling Fops, base Loons, saucy Coxcombs, idle Lusks, scoffing Braggards, noddy Meacocks, blockish Grut|nols, Doddipol Iolt heads, jobernol Goosecaps, foolish Loggerheads, slutch Calf lollies, grout-head Gnatsnapper, Lob dotterels, gaping Changelings, codshead Loobies, woodcock Slangams, ninny-hammer Flycatchers, noddipeak Simpletons, turgy Gut, shitten Shepherds, and other such defamatory Epithetes….
— François Rabelais, (trans. by T. Urquhart), The works of F. Rabelais, 1694

Definition - dullard

In stark contrast to the earlier words on this list, dulbert is not formed by combining a word with head, or one of its synonyms; it is thought to have been made by combining a word (dull) with a word for beard.

…unlesse you hold al men Dulberts like your rare workman of Winchester, who our master work men of London account a very bungler….
— Mark Ridley, Magneticall animadversions made by Marke Ridley, 1617

Definition - numskull, nincompoop

Jobbernowl is not yet considered archaic, although it is encountered far less frequently than most of its synonyms (and mostly is found in British use). The origins of the word are somewhat in question, but it is believed to come in part from the Old Testament patriarch Job, making it one of our few words for “nincompoop” that come of biblical roots.

Thy father looking one way, and thy mother,
For feare of being spide, she look'd another;
And leering sundry waies, kept carefull watch,
Lest any at their businesse should them catch.
And that's the reason why thine eies doe rowle,
And squint so in thy, doltish iobbernowle.
— John Taylor, All the workes of Iohn Taylor, 1630