Definition: being a dunderhead
What is a dunder, and why do we use this word to insult people in a somewhat mild fashion? Alas, this is but one more unknowable in a long line of things that etymology has not yet figured out. It may come from the Dutch word for “thunder” (donder), but no one is sure about that. What we are sure about is that there are a great number of words for “stupid” or “stupid person” ending in -headed or head. A partial list includes the following: airhead, beefheaded, beetleheaded, blockhead, bonehead, bullhead, bubblehead, buffle-headed, butthead, chucklehead, deadhead, fathead, flathead, hammerhead, heavy-headed, idleheaded, ironhead, jolt-headed, jughead, knucklehead, loggerheaded, lunkhead, meathead, muddlehead, pinhead, pinheaded, ramhead, saphead, sheepheaded, sheepshead, thickhead, and wooden-headed.
Thus I pass away the time, and this lulls me on to drudge on the end of the Chapter with this Dunderheaded Master of mine, who to my knowledge is more a Madman than a Knight.
— Miguel de Cervantes, Don Quixote (anon. trans.), 1703