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1. deliberately cruel or violent:

〈人･行為などが〉残忍な, 残虐な, 乱暴な, むごい

e.g. a vicious assault.

(of an animal) wild and dangerous to people.

〈動物が〉獰猛（どうもう）な

serious or dangerous:

｟くだけて｠ 〈天候などが〉ひどい, 激しい; 〈痛みなどが〉強烈な

e.g. a vicious flu bug.

2. literary immoral:

｟詩/文｠ 不道徳な, 堕落した(↔ virtuous)

e.g. every soul on earth, virtuous or vicious, shall perish.

3. archaic (of language or a line of reasoning) imperfect; defective.

｟古｠ 〈推論などが〉欠陥のある, 不完全な.

Middle English (in the sense ‘characterized by immorality’): from Old French vicious or Latin vitiosus, from vitium ‘vise’.