R.W. Burchfield, The New Fowler's Modern English Usage
(Oxford: Clarendon Press, 1996), s.v. decimate
A key word in the continuing battle between prescriptive and descriptive linguists. L. decimāre meant '(mil.) to punish every tenth man chosen by lot'. 'Punish' sometimes meant 'put to death'. In strict terms, therefore, decimate should mean 'to kill, destroy, or remove one in every ten of (something)', and it has been so used in English since the 17c. Rhetorically and loosely, as the OED expresses it, it has also been used from about the same time to mean 'to destroy or remove a large proportion of; to subject to severe loss, slaughter, or mortality'. The dispute between those who will and those who will not use decimate in its 'rhetorical or loose' sense is unresolved.
Polybius 6.38 (tr. W.R. Paton):
If the same thing ever happens to large bodies, and if entire maniples desert their posts when exceedingly hard pressed, the officers refrain from inflicting the bastinado or the death penalty on all, but find a solution of the difficulty which is both salutary and terror-striking. The tribune assembles the legion, and brings up those guilty of leaving the ranks, reproaches them sharply, and finally chooses by lots sometimes five, sometimes eight, sometimes twenty of the offenders, so adjusting the number thus chosen that they form as near as possible the tenth part of those guilty of cowardice. Those on whom the lot falls are bastinadoed mercilessly in the manner above described; the rest receive rations of barley instead of wheat and are ordered to encamp outside the camp on an unprotected spot. As therefore the danger and dread of drawing the fatal lot affects all equally, as it is uncertain on whom it will fall; and as the public disgrace of receiving barley rations falls on all alike, this practice is that best calculated both to inspire fear and to correct the mischief.
Plutarch, Life of Crassus
10.2-3 (tr. Bernadotte Perrin):
Five hundred of them, moreover, who had shown the greatest cowardice and been first to fly, he divided into fifty decades, and put to death one from each decade, on whom the lot fell, thus reviving, after the lapse of many years, an ancient mode of punishing the soldiers. For disgrace also attaches to this manner of death, and many horrible and repulsive features attend the punishment, which the whole army witnesses.