A knucklehead is someone who is a dunce or does something foolish. You might criticize your friend's baking skills by saying, "You used salt instead of sugar, you knucklehead!" Knucklehead is an informal term that can come across as joking rather than insulting, depending how you use it.

What does knucklehead mean?

British Dictionary definitions for knucklehead

knucklehead. / (ˈnʌkəlˌhɛd) / noun. informal fool; idiot.

Why are people called knuckleheads?

knucklehead (n.)

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.