I had that feeling twice in particular - once when I connected hippopotamus and Mesopotamia - realising that the first is a river horse, and if that ending means river, Mesopotamia must mean the place between rivers. And it is named because of Tigris and Euphrates. That suddenly clicked into place.
Second when it dawned on me when I was an embarrassingly high age that gendarmerie and gendarme simply came from a contraction of gens d'armes, men of arms.