In a disrespectful, impolite, or offensive manner that shows a lack of courtesy toward others.
From 'rude' (from Latin 'rudis' meaning rough, coarse, or unrefined) plus the adverb suffix '-ly'. The sense evolved from 'physically rough' to 'socially rough' or impolite.
Calling someone 'rude' is actually a relatively recent concept in human history—ancient Romans used 'rudis' to mean physically crude, but the modern idea of 'rudeness' as a social crime really took off with the rise of courtly manners in medieval times!
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