The quality of being unpleasant, disgusting, mean-spirited, or morally repugnant.
From English 'nasty' (possibly from Middle Dutch 'nestig' or Old French roots) plus '-ness' (suffix creating nouns from adjectives). The word gained prominence in the 16th century.
The word 'nasty' has changed meaning over time—it once meant 'lowborn' or 'ignoble,' then shifted to mean 'physically disgusting,' and now can mean 'morally repugnant,' showing how insults evolve as society changes what it considers most offensive.
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