It was a punishment meted out to Roman troops where one out of every ten men in a unit, say ten men out of a Century (or 400 out of forty centuries, or a full Legion), was selected at random and executed. For mutiny, disobeying orders, cowardice, etc. The French army in WWI followed a similar policy against units that refused orders to leave their trenches in suicidal attacks against German barbed wire and machine guns. It reduced the unit’s strength by ten percent, but presumably the remaining ninety percent would have gotten the message. Now it has no specific meaning except for some kind of horrendous slaughter.
The Italian army in WWI also resorted to decimation as a disciplinary tactic.
Words change meanings over time, and this one is no different. Imagine in 100 years what the word ‘marriage’ will entail....................
Please be careful with generalities. The French Army did NOT practice what you are suggesting. In World War I the was a short Mutiny after the disaster of the Chemin des Dames. Whereas several hundred were charged with failure to perform their duty, or even desertion, very few (somewhere around fifty) were ultimately executed. They were revolting not only against the senseless “over the top” orders, but also against the maltreatment they received on a regular basis. This led to the appointment of General Petain who changed many of the customary conditions of the men and was able to pull the French Army back together. This was the same Petain who in WW II capitulated to the Nazis and “ruled” France under the Vichy government.
You are absolutely correct.