Knuckles

I wonder where the expression, knuckle-head" came from? Is it related to knuckle-sandwich?

Knuckle comes to us from Middle English, somehow related to the Old High German word for knot. It is the rounded prominence formed by the ends of 2 adjacent bones at a joint. And a few other things that somehow resemble a knuckle or the prominence at an intersection.

Knucklehead means dumbbell according to my Dictionary, another interesting compound word. This could be confusing, as dumbbell has 2 meanings. It would be strange to hear someone at the health club looking for some knuckles!

Knuckle-sandwich is not in my dictionary. I think the meaning is clear, but there could be people who speak English as a second language and they might not understand that it means a punch to the mouth.