Old English = Legioun
Old French = Legion
French = L'egion
Latin = Legio
The first I heard of it was in reference to the Roman Legions. They were set up like the Greeks, by ranks and files, and stood in formation during the attack, or attacked in mass as one entity. Very formal when you consider that most people fought in disarray, like thousands of individual free-for-alls.
Hence, it came to also mean 'many.' And, in the Bible, it is used as the name of a demon, that inhabited one man. Thus, when the demon spoke says his name was Legion, he was saying that the man was plagued by more than one demon, perhaps thousands. And, like STDs, most people who suffer such afflictions have more than one. The term has lived on in English and other languages, thanks to this usage.
In war, the tactics they used came and went. The Templars were the most formal group to use it, and then later, with ranged weapons, it came back until recently. This stupidity should have ended around the time of the American Civil War, but lingered for a while afterward. Today, they simple give one man a full-automatic weapon and expect him to fire as much as an army, and still hit something. ;)