A stupid or stubborn person; a blockhead or someone hard-headed and difficult to convince.
Compound word from boulder (hard, unyielding rock) plus head, metaphorically comparing a stubborn person's inflexible mind to a solid rock.
The insult 'boulderhead' is poetic—calling someone as hard and immovable as a rock is a metaphor that probably dates back to when people actually lived among boulders.
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