Synonyms for Knucklehead:

n.

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

Other synonyms:

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