A stupid or foolish person; a blockhead, similar to 'dunderhead.'
From 'dunder' (of uncertain origin, possibly Dutch or Low German, meaning 'stupid' or relating to thunder/noise) combined with 'bolt' (something sudden or swift). The compound suggests someone thick or dense as a bolt of wood.
English loves piling insults together into compound words—'dunderbolt' combines two weird elements ('dunder' + 'bolt') to create something that sounds both aggressive and comical at the same time.
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