In a rude, impolite, or ill-mannered way that shows a lack of respect for others.
From 'boorish' (adjective, meaning crude or ill-mannered) plus the adverbial suffix '-ly'. The root 'boor' comes from Middle Dutch 'boer' meaning farmer or peasant, and originally carried a class-based stigma in 16th-century English.
The word 'boorish' started as a class insult—Dutch farmers were stereotyped as crude by city dwellers—but now it's purely about behavior, not background. Language often preserves old prejudices even after we've forgotten why!
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