A stupid or foolish person; a blockhead or idiot.
From 'dunder' (possibly from Dutch 'donder,' meaning thunder, or from a Scandinavian word meaning 'stupid') combined with 'head' (meaning the seat of intelligence). The compound suggests someone whose head is full of noise rather than sense.
'Dunderhead' is one of those perfect insults where the sound matches the meaning—'dunder' itself sounds dumb, so sticking 'head' on the end makes it even more dismissive and fun to say.
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