Definition of decimate:
(v) :
to kill one man chosen by lot out of every ten in a legion or other military group; to reduce anything by one in ten, or ten percent
"ca. 1650: Jeremy Taylor, (from vol. 1 of an 1835 edition of his works) - God sometimes decimates or tithes delinquent persons, and they died for a common crime, according as God hath cast their lot in the decrees of predestination."