- Implying size or importance.
Probably earlier than the 15th Century. From Latin, magnus, meaning great. Also seen in Old Norse, mageus, Old English, maegen, and Old French, maine, as well. All meaning the same thing.
A term applied to many things aboard all types of vessels. For example, mainmast, mainsail, main engines, main hatch, etc. Pretty self-explanatory.
Rogers, J.G. (1985). Origins of Sea Terms. Mystic, CT: Mystic Seaport Museum.