Decimate is one of the most egregiously misused words today. The difference between intended and actual meaning is almost as great as with literally/figuratively, the true meaning is right there in the word, and the historical use is pretty amazing and not to be forgotten.
Modern usage is typically that to decimate means to drastically reduce, similar to annihilate. In the historical usage decimate meant to reduce by 1/10th, actually a fairly small reduction.
Language is a fluid and evolving thing, and words may change meaning over time, but this example seems unfortunate.
Note that "decim" in decimate? As with decimal, or decibel, it comes from the Latin word decimus, meaning 1/10th. Note decibel actually means 1/10th of a Bel, the actual unit of sound intensity, named for Alexander Graham Bell. Since the original meaning can be guessed just from it’s spelling, it’s a bit confounding that it has managed to drift so drastically. Note that annihilate is more true to it’s construction, with that Latin "nil" inside meaning “nothing.”
The final reason the misuse is a shame is that the original use is a remarkable and more unique idea. About 25 centuries ago, the Roman army came up with a clever and horrible punishment for mass desertion. Typically, desertion would be punished by death, but they did not want to lose their own soldiers en mass. And, these being fighting men, such a punishment may be difficult to enact on a large group. The solution was to randomly sentence one in ten of the offenders to death. The whole mass would feel the real fear of punishment, all would feel the loss of people they knew personally and take it to heart, and those spared could be counted on to not rebel, and even to help enact the sentence under orders.
That punishment has been applied as late as the first world war. The word was also used to refer to the tithe (typically 1/10th of one’s income) staring probably in the 1600’s, and the latest use as referring to a massive reduction seems to have arisen in just the last 100 years or so.
