Extremely stubborn and stupid; unable or unwilling to understand something obvious or change your mind.
Compound of 'block' (something solid, impenetrable) and 'head' (mind/intellect). Dating to around 1600s, it compares a dull mind to a solid block of wood that nothing can penetrate.
Calling someone 'blockheaded' is insulting because it suggests their mind is literally as empty and solid as a wooden block—ironically, this insult is nearly 400 years old and still works perfectly.
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