Wordorigins.org

Port, larboard, and starboard are nautical terms of direction referring to the left (port, larboard) and right (starboard) of a ship as one faces the bow. The oldest of these terms is starboard.

Starboard has a simple and straightforward, if not particularly obvious to the modern speaker, origin. It comes from Old English stéorbord and is a combination of stéor, meaning steer, and bord, meaning the side of a ship or boat (also found in the modern overboard). On old ships the rudder or steering paddle would be on the right side of the ship. Hence the term.