A cohort (roughly 480 soldiers) selected for punishment by decimation was divided into groups of ten; each group drew lots (sortition), and the soldier on whom the lot fell was executed by his nine comrades, often by stoning or clubbing. The remaining soldiers were often given rations of barley instead of wheat (the latter being the standard soldier's diet) for a few days, and required to camp outside the fortified security of the marching camp.Because the punishment fell by lot, all soldiers in the group were eligible for execution, regardless of the individual degree of fault, or rank and distinction, unless rigged to eliminate the mutiny ringleaders. The leadership was usually executed independently of the one in ten deaths of the rank and file.
I'm not unsympathetic with the goal of using words correctly but:
Languages evolve over time and lots of words have changed from their original meanings and/or added new meanings. We don't have to keep everything the way that it was in ancient Rome. When literally every dictionary says that a particular word usage is OK, it's OK. When 90% of people start using a word in a new way the meaning has effectively changed.
According to the Oxford English Dictionary there have been two meanings to the word decimate since the mid-1600's and people have been arguing about this since 1870. link1, link2
By the way, lately I have been reading a lot about Chinese etymology. Many Chinese characters have changed their meaning over time.