A foolish or stupid person; a blockhead (similar to dunderhead).
From 'dunder' (meaning stupid or confused) combined with 'pate' (an archaic or dialect word for head, from Latin 'pate'). The compound is a variant of 'dunderhead' using a different word for the head.
'Pate' is one of those old-fashioned words for head that survives mainly in insults and formal speech—'dunderpate' shows how English used to have multiple synonyms for stupid heads, each with slightly different regional flavor.
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