Killed, destroyed, or severely damaged a large portion of something, originally meaning to kill every tenth person.
From Latin 'decimatus,' derived from 'decem' (ten). Roman armies used this as actual punishment—killing every 10th soldier in a mutinous unit—so the word carries its original violent meaning.
Even though people now use 'decimated' to mean anything badly damaged, it originally meant specifically ONE-TENTH destruction, which is actually way less severe than what people usually mean—the word's meaning has gotten more dramatic over time.
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