The Roman punishment behind "decimate" is famous. It was also, by the record, very rare.
The word comes from Latin decimare, built on decem, "ten." It named a sentence passed on a unit that had fled or mutinied. The Greek historian Polybius describes the procedure: the men were assembled, one in ten was chosen by lot, and their comrades killed them with clubs. The survivors were fed barley and made to camp outside the fortified walls.
For all its fame, Wikipedia's account counts only eleven recorded instances. The earliest claimed case, in 471 BC, comes from Livy and is probably not historical. Marcus Licinius Crassus used it in 72 BC against a unit that fled during the Third Servile War, killing about fifty men. Mark Antony and Octavian each used it in the 30s BC, and Galba in AD 69.
Historians also doubt the ancient writers' numbers. Literary accounts tend to exaggerate, and one scholar has called decimation "essentially a rhetorical myth." Polybius's own narrative suggests fewer than twenty victims in a typical case. Professional soldiers, as another historian put it, could hardly be expected to execute their own comrades at random.
English borrowed the word around 1600 in the strict sense: to choose by lot and put to death every tenth man. By the 1660s it was already being used, as Etymonline puts it, "loosely and unetymologically" for destroying a large but indefinite number. That is the meaning nearly everyone uses today. Etymonline notes that it still irritates pedants.
There was briefly a third sense. In 1821, John Quincy Adams used "decimate" in a report on weights and measures to mean making a scale decimal.
The punishment itself outlived Rome. The Italian general Luigi Cadorna used it in the First World War. In 1917 men of the Catanzaro brigade were shot in two groups, sixteen and then twelve.