Third-person singular present: to destroy or kill a large portion of something, or historically, to kill every tenth person as punishment.
From Latin 'decimatus' (killed every tenth), from 'decimus' (tenth). Originally a specific Roman military punishment, the meaning broadened to mean severe destruction of any large group.
Roman generals would decimate mutinous legions by literally executing every tenth soldier chosen by lottery—it was brutal enough to restore discipline but not so severe as to destroy the entire army's fighting capacity.
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