Or if you're not studying anything, just a random fact will do. But try and make sure they are facts. Don't want to be giving people false information. No need to cite anything, we're not fascists here.
The word Decimate (Latin) dervies from an Ancient Roman Republican form of punishment. (I'm not certain if it was for cohorts or entire legions but I'd image it would be for cohorts only.)
The punishment was used only in the most extreme circumstances to enforce discpline. The soldiers were lined up in a random fashion. And every tenth man (Deci) was forced to be beaten to death by his comrades. This continued through the entire (cohort?) until every tenth man had been killed.
Presumably this was used if the Standards were lost (They were rarely ever lost so it is unlikely). But it was mainly used for cowardice.