"Decimate" is one of the few English words that began with arithmetic precision. The Latin decimare meant "to take a tenth," from decimus ("tenth"), itself from decem ("ten" — the same root as decade, December, and decimal).
A Roman Punishment
In the Roman army, decimatio was a disciplinary measure used against units that had mutinied or shown cowardice. The men were divided into groups of ten and forced to draw lots; the unlucky one in each group was clubbed or stoned to death by the others. The remaining nine were given rations of barley instead of wheat and made to sleep outside the camp until they had redeemed themselves. The point was both punishment and deterrent: the survivors lived with the memory of having killed a comrade.
From Tenth to Tithe
The same root carried over into peacetime. In the medieval Church, decimare meant "to collect a tenth," and that is where the English word tithe ultimately comes from — though by a different Germanic route. To "decimate" income or grain was to take a 10% tax for the Church or the state. Both senses — the punitive and the fiscal — were available to English writers in the 17th century when the verb was first borrowed.
A Word That Lost Its Tenth
By the 19th century, English speakers were using "decimate" to mean simply "destroy a large portion of" — with no attention to the original 10%. Today the word is more often used to mean "almost wiped out" than "reduced by ten percent." Strict usage guides still object, pointing to the precise Latin meaning, but the broadened sense is now standard. The word has, fittingly, decimated its own etymology.