Definition of "dunderhead"

You're the stupidest dunderhead I ever saw or ever heard of, so help me Moses!

1883, Mark Twain [pseudonym; Samuel Langhorne Clemens], “A Cub-pilot's Experience”, in Life on the Mississippi, Boston, Mass.: James R[ipley] Osgood and Company

Poor old fellow's a dunderhead. That's where it is in a nutshell. Never could make a living. […] Nice old chap as ever lived. Only impractical, dreamy. Gentle as a sheep—and no more capable of running that big, expensive plant than a motherly old ewe.

1915, Basil King, chapter 3, in The Side Of The Angels

Cheney & Company swooned over Mr. Chalabi because he was telling them what they wanted to hear. . A half-dozen dunderheads who thought they knew everything assumed they could control Mr. Chalabi and use him as the instrument of their utopian fantasies.

2004 May 23, Maureen Dowd, “Bay of Goats”, in New York Times, retrieved 29 November 2017