What does "knucklehead" mean?

An idiot; a stupid or inept person.

What is a synonym for "knucklehead"?

Common synonyms include: blockhead, dunce, addlebrain, addlehead, addlepate, beefhead.

Pronounced //ˈnʌkl̩ˌhɛd//.

/ˈnʌkl̩ˌhɛd/ noun, slang

noun, slang ·Moderate ·High school level

Noun

1
An idiot; a stupid or inept person. derogatory, slang
"Don't stick that screwdriver into the live electrical outlet, you knucklehead!"
2
a stupid person; these words are used to express a low opinion of someone's intelligence wordnet

"Only a knucklehead would do that."

From knuckle + head.