'Decimate' originally meant killing exactly one soldier in ten — a Roman military punishment by lot.
To destroy a large proportion of something; historically, to kill one in every ten soldiers as a punishment for mutiny or cowardice in the Roman army.
From Latin 'decimāre' (to take a tenth, to tithe; in military contexts, to select every tenth soldier for execution), from 'decimus' (tenth), from 'decem' (ten), from PIE *déḱm̥ (ten). The Roman military practice of decimation (decimātiō) involved selecting soldiers by lot and having the nine survivors beat the tenth to death. The modern sense of 'to destroy a large portion' — rather than precisely one
Roman decimation was terrifyingly random: the condemned legion was divided into groups of ten, each group drew lots, and the unlucky soldier was beaten to death by the other nine — who were his own comrades and tent-mates. The historian Polybius records that the practice was used to punish entire units for cowardice, and Marcus Licinius Crassus famously decimated his legions during the Spartacus revolt in 71 BCE.