Stupid; foolish; displaying the characteristics of a dunderhead.
From 'dunderhead' with the adjectival suffix '-ed,' which turns nouns into adjectives (as in 'hot-headed,' 'bull-headed'). The suffix transforms the noun into a descriptive property.
When you add '-ed' to 'dunderhead,' you're converting an insult from a label for a person into a description of how someone behaves—'dunderheaded' lets you attack the foolish action rather than the person.
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