Green’s Dictionary of Slang

blockhead adj.

[blockhead n.1 ]

stupid, foolish.

[UK]T. More Utopia II (1624) 82: A lumpish block-head churle [...] which hath no more wit then an asse.
[UK] ‘Three Children Sliding on the Thames’ in Playford Pills to Purge Melancholy II 6: May London say, Woe worth the Carpenter, / And all such Block-head fools.
[UK] in D’Urfey Pills to Purge Melancholy IV 2: [as cit. 1700].
[US]C. Hiaasen Lucky You 305: He’d had to play hardball with those blockhead wholesalers back in Wisconsin.