knucklehead (n.)

also knuckle-head, "stupid person," 1890, American English, from knuckle (n.) + head (n.).

"That infernal knuckle-head at the camp ought to have reported before now," he thought to himself, as he smoked. [Charles H. Shinn, "The Quicksands of Toro," in Belford's Magazine, vol. v, June-November 1890, New York]

From 1869 as the name of a part in a type of mechanical coupling device. Popularized in the "stupid person" sense from 1942, from character R.F. Knucklehead, star of "Don't" posters hung up at U.S. Army Air Force training fields.

Everything Knucklehead does is wrong and ends in disaster. He endures one spectacular crash after another so that the students at the Gulf Coast Air Force Training Center may profit by his mistakes, and it looks now as if there will be no let-up in his agony. [Life magazine, May 25, 1942]