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.