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also block-head, "stupid person," 1540s (implied in blockheaded), from block (n.) + head (n.); probably originally an image of the head-shaped oaken block used by hat-makers, though the insulting sense is the older one.
A stupid person; klutz: No man but a blockhead ever wrote except for money
[mid-1500s+; that is, a head no more intelligent than the block on which hats are made]