Vicious describes behavior that is cruel, violent, or very harmful. It can also describe cycles or attacks that keep causing more damage or trouble.
From Old French 'vicios', from Latin 'vitiosus', meaning 'full of faults or defects', from 'vitium' meaning 'fault or vice'. The sense shifted from moral faults to more physical cruelty and harm.
The original idea behind ‘vicious’ was being full of vice—moral flaws—before it came to mean outright cruel. That’s why we still say ‘vicious rumors’ even though they don’t bite like animals. The word treats meanness as a kind of moral attack.
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