leŭtenanto

Cut it the obvious Esperanto way and you get leŭ plus tenanto, one who holds. That is very nearly right, which is why it earns an explanation rather than a warning. French lieutenant really is lieu (place) plus tenant (holding): the officer who holds the place of his superior when the superior is not there. It is a calque of Latin locum tenens, which is where English gets locum for the doctor covering somebody's practice. So the seam is real, and place-holder is exactly what the word means.

In Esperanto, though, it is a single lump. Leŭ- is not a morpheme, there is no root meaning place hiding inside it, and you cannot build anything else out of it. This is the same class as inspiri, where the Latin pieces are perfectly visible to a Latinist and perfectly dead to the language: enjoy the etymology, then file the word as an unanalysable borrowing.

For the ranks themselves, Esperanto borrows the international set wholesale: a leŭtenanto is junior to a kapitano, who is junior to a kolonelo, and all three are oficiroj.