In a manner that is savage, cruel, or uncivilized; extremely harshly or brutally.
From 'barbarous' (adjective) + '-ly' adverbial suffix. 'Barbarous' derives from Latin 'barbarus,' which originally meant 'foreign' but came to mean 'savage' or 'uncivilized' due to Greek and Roman attitudes toward non-Mediterranean cultures.
Medieval Europeans used 'barbarous' to describe Vikings, but the Vikings were actually incredibly sophisticated—they had complex laws, art, and navigation skills! The word says more about the prejudice of the people using it than the people being described.
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