The Etymology of Marine
Marine reached English in the late Middle Ages by way of Old French marin, the adjectival form of Latin mare meaning sea. The Latin root traces back to Proto-Indo-European *móri, a word for any standing body of water; the same root produced Old English mere (a lake) and survives in place-names like Windermere. As an adjective, marine refers broadly to anything of the sea — marine biology, marine insurance, marine engines — and contrasts neatly with terrestrial. The military noun sense is younger and French-flavoured: in the 17th century the French navy maintained troupes de la marine, soldiers stationed aboard ships to act as boarding parties and guards. England formed equivalent companies in 1664, and by 1672 they were simply marines. The United States Marine Corps preserves the word in its modern soldier-of-the-sea sense. So the adjective and the noun share one root but two careers, one studying the ocean and one fighting on it.