Foolish, stupid, or prone to making blunders; having the characteristics of a blunderhead.
From 'blunderhead' plus the adjectival suffix '-ed,' used to indicate possession of a quality (as in 'big-headed' or 'hard-headed'). The '-ed' here doesn't indicate past tense but rather means 'having' or 'characterized by.'
When English adds '-ed' to a noun like 'blunderhead,' it transforms the noun into an adjective meaning 'characterized by that quality'—so a 'blunderheaded' person is one with the nature of a blunderhead.
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