From Middle English ladebord, referring to the side of the ship on which cargo was loaded. Changed to larboard in the 16th century by association with starboard.
Usage notes 
In the British Navy it was not until 1844 that larboard was abandoned for port in reference to that side of the ship. The term port however had always been used when referring to the helm (ie. sailing direction), in order to avoid any confusion between starboard and larboard in such an important matter. (Reference: Ray Parkin, H. M. Bark Endeavour, Miegunyah Press, second edition 2003, ISBN 0-522-85093-6, page 56.)