In a way that is stubborn and shows stupidity or refusal to understand something obvious.
Adverbial form created by adding '-ly' to 'blockheaded,' following standard English grammar for converting adjectives describing manner into adverbs.
Adding '-ly' to insults is a uniquely English thing—we can turn almost any criticism into an adverb, turning 'blockheaded' into 'blockheadedly,' like we're scientifically describing someone's stupidity.
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