Synonyms for Knucklehead:

n.

joker, airhead, clown, lump, maniac, numpty, dumb blond, fool. blockhead (noun)
dunce.
dunce (noun)
blockhead, fool, dunderhead, numskull, bonehead.
knucklehead (noun)
dunce, blockhead, dunderhead, lunkhead, numskull, hammerhead, muttonhead, bonehead, loggerhead.
stupid person (noun)
dunce.

Other synonyms:

Other relevant words:
fool, maniac, blockhead, joker, lump, numpty, airhead, clown.