"knucklehead" is a correct and viable word in written English and can be used in a casual context.
You might use it, for example, to describe someone who makes an irresponsible or foolish decision. For example, "John is such a knucklehead - he forgot to pay his rent on time again!".
Oh, and occasionally, to swear violently at the knucklehead who's trying to drive his car through my right leg.
"I was a knucklehead in my time, but now I'm out here doing this for the community".
"Look at that fat-ass knucklehead," he says in Brooklynese as thick as an Italian grandpa's.
So not only will you be freezing your tits off in the Outer Hebrides, you'll have to do it while some absolute knucklehead is lecturing you about paradigms and Che Guevara.
What could be better, for his purposes, than a boss with eyes only for the football field, who dismissed him as an exasperating, impulsive knucklehead?
Christie continued, "Any knucklehead neighbor of yours who's telling you, 'Uh-oh, you sign that easement, Christie's comin' in here buildin' showers for people, a bathroom, a hot-dog stand, before you know it there's going to be a Dairy Queen'... Let me use a word that's indelicate".
He was the untutored knucklehead ("Is Moby Dick the man or the whale?" he famously asked) who lived on unfiltered cigarettes, poker chips, and Scotch and yet somehow managed to hire James Thurber and E. B. White, Janet Flanner and Lillian Ross, Edmund Wilson and Vladimir Nabokov, A. J. Liebling and Joseph Mitchell.
Pineda wasn't a cheater, simply a knucklehead, and, as the subtext of this conversation implied, he was ruining a good thing for all the other pitchers in the league.
He must've pegged me for a knucklehead right off the bat, but I didn't know it until I turned in my term paper.
"People always say they want to lose ten pounds, but I feel like that ten pounds could be where all my humanity and humor is, and if I lose it I might just be a skinny knucklehead," she said.
The News called Koch a "knucklehead... possessed of a rare combination of gullibility and cavalier indifference to money".
