A knucklehead is a blockhead, a stupid person, a dumbbell, etc.
But why's a knucklehead a knucklehead? Is it because one envisions taking one's knuckles and hitting on the knucklehead's head in order to wake up or stir up some activity beneath the bonehead's skull?
Just curious,
DubDub
Dear WW: Diligent search uncovered only junk. My interpretation is that since knuckles are mostly dense bone with no room for even a tiny brain, it means the person so described is a brainless bonehead.
I was guessing it had something to do with a person's preference for bare-knuckle brawn over brain, but that makes sense, too.
k
it seems as though there are innumerable words of this ilk -- one of my favorites is 'gormless'.
http://home.mn.rr.com/wwftd/
Whaaaa---a-at?
- Joe (I wish it was) Friday
This from
http://makeashorterlink.com/?F26941C8 (not sure of the credibility of the info but sounds okay - it's from a site that teaches english on the net).
knucklehead Definition: A stupid person.
Example: 1) Charlie's a real knucklehead -- he's been in first grade for five years! 2) You knucklehead! How could you have mixed the the dog food with the cereal?
Etymology: A 'knuckle' is a joint or a place where bones meet, particularly the bones of the hand. This word suggests that someone's head looks like or has been hit by a hand, thus making the person stupid. And for whatever reason, the phrase just sounds funny.
That's the best I could come up with DubDub...
Hev
DubDub,
Knucklehead is also the name given to the 1936 Harley Davidson engine. If interested in info:
http://www.tower.org/museum/harley_davidson/harley_davidson.htmlI'm a motorcyclist myself, although not a Harley enthusiast, but I know it is a very common term when describing Harleys.
The machines, not the riders, as some would guess! 
Thanks, everybody. I like wwh's theory about the density of the knuckles and their not being able to contain even a tiny brain! And, Angel, I'm pretty sure I'll forget the Harley information since I pretty much forget everything put before me that has to do with the mechanical--but, heh, it was an interesting read!
Best regards,
Dub
Knuckle comes from ME knokel, akin to D kneukel, G Knochel, dim. of a word represented by D knok, G Knochen, meaning "bone," so I surmise that knucklehead is just another way of saying, as Dr Bill's post hints, bonehead.
BTW - knuckle down comes from the game of marbles, referring to the concentration of purpose manifested in a player who is on the floor, knuckle against the ground, preparing a shot, and knuckle under comes from the idea of bowing, knee to floor, in submission.
Bravo, Sparteye! Knucklehead = Bonehead! Gotcha!
Boneheaded regards,
WoodyWind