Have you ever noticed that the terms for military ranks are pretty inconsistent in terms of how sensible they seem to be? At the extremes, “private” and “general” could be assumed to refer to scope of responsibility. If you’re a “private”, you’re responsible only for yourself (and maybe not even that), while a “general” is responsible (“in general”) for a whole military force.
But the ranks in between are less clear. Take, for example, “lieutenant.” The first part sounds like “in lieu of,” because that’s exactly what it is. It’s originally a French word, and entered English after the Norman invasion. Its literal meaning is “place” (lieu) “holding” (tenant), and it comes from the idea that a “lieutenant” was standing in or acting in the place of someone else — in this case, a superior officer.
“Lieutenant” is occasionally used as a generic, nonmilitary term. You might read about the leader of one or another sort of organization (business, religious, political, etc.) having “lieutenants.” This isn’t usually meant to imply anything military; it’s just that a “lieutenant” really is just someone who acts in another’s place.
None of that explains why “lieutenant” is pronounced “lef-tenant” in England but “loo-tenant” in the US. Nobody really knows, but the best guess is that maybe a few centuries ago, people mistook the “u” in the first syllable as a “v”. Doesn’t seem all that plausible, but it’s the only theory we’ve got.
Other military ranks also come from French. “Sergeant”, for example, is from the Old French “sergent,” which meant “servant.” That sense has been obsolete in English for at least a couple of centuries. In terms of military usage, “sergeant” originally meant what “private” does today; a low ranking or common soldier who mostly proceeds on foot. That, too, was derived from “servant” — because one of your servants might be a “footman.”
A “colonel,” in contrast to sergeants and lieutenants, is a pretty high rank. Not high enough, though, to escape similarly French origins. The origin of the meaning of “colonel” comes from the way armies used to fight wars by marching in “columns” — a “colonel” was the leader of a “column.” As for the weird pronunciation (“colonel” is pronounced “kernel”, which seems to make no sense at all), this also goes back to the word’s French origins. In French, back in the 1500s, the guy at the head of the column was a “coronell.” The “r” morphed into an “l” as part of some sort of general linguistic shift in the late 1500s.
As for “corporal”, its origin is a bit of a mystery. It’s another term borrowed from French, and at about the same time as the others, but although it can be traced back to “corporal” in French and “caporal” in Italian, the trail goes cold at that point. Nowadays a “corporal” is an army rank, but in the 1600s the petty officer on a ship was called a corporal too.
It seems all the privates and sergeants have to get ordered around by somebody though, and whether that somebody is a corporal, a colonel, or another rank, it’s really a lieutenant speaking in lieu of a general.
