Green’s Dictionary of Slang

knucklehead n.

also nuckel-head
[knuckles pressed to the forehead imply the intensity of thought for one who is not overly bright]

1. (orig. US) a term of abuse, a description for any foolish, stupid, slow person.

Nevada State Journal 26 June n.p.: Wm. ‘Bill’ Hopper, president of the First National Bank of Nevada, has decided to drag the novice or greenpea out into the open where all may take a look and to the tune of a beautiful trophy and fancy cash prize, but ‘ye knucklehead’ must pay an entry fee to show his skill in the arena.
[US]G. McMillan Old Breed 241: This is the way to do it, knucklehead!
[US](con. 1950) E. Frankel Band of Brothers 2: You stupid knuckleheads [...] You ever point a weapon at me again, I’ll have you rotatin’ on the end of it!
[US]G.L. Coon Meanwhile, Back at the Front (1962) 36: I want you to hang that dumb bastard, that’s what I want you to do about it, you knucklehead!
[US]L. Rosten Dear ‘Herm’ 43: Even my son Alvin agrees – and he is no nuckel-head!
[US]D. Woodrell Muscle for the Wing 105: He’s a knucklehead.
[US]G. Sikes 8 Ball Chicks (1998) 75: The knucklehead who got shot is named Evil Dog.
[US]G. Pelecanos Soul Circus 208: Doesn’t tolerate knuckleheads or any kind of foolishness.
[US]Burns & Corthron ‘Misgivings’ Wire ser. 4 ep. 10 [TV script] You’ve always been good with the knuckleheads, boss.
[US]A. Steinberg Running the Books 155: The girl over there wants to sue you for being a knucklehead.

2. in attrib. use of sense 1.

[US]J. Díaz This Is How You Lose Her 3: Instead of us hanging with my knucklehead boys [...] we were seeing movies.