Old English steorbord, literally "steer-board, side on which a vessel was steered," from steor "rudder, steering paddle," from Proto-Germanic *steuro "a steering" (compare German Steuer), from PIE *steu-, secondary form of root *sta- "to stand, make or be firm," + bord "ship's side" (see board (n.2)). Similar formation in Old Norse stjornborði, Low German stürbord, Dutch stuurboord, German Steuerbord.
Early Germanic peoples' boats were propelled and steered by a paddle on the right side. The opposite side of the ship sometimes in Germanic was the "back-board" (Old English bæcbord). French tribord (Old French estribord), Italian stribordo "starboard" are Germanic loan-words.