Middle English ladebord (see lade¹, board), referring to the side on which cargo was put aboard. The change to lar- in the 16th century was due to association with starboard.

• port from Old English:

  Latin portus ‘haven or harbour’ is the source of our word port. Its nautical use to refer to the left side of a ship, the opposite of starboard (OE from ‘steer board’—early ships were steered with a paddle over the right side), dates from the mid 16th century and probably comes from the idea that this was the side of the ship where the loading hatch was fitted and was turned towards the quay when the ship was in port. It replaced an older word larboard, hardly surprising given the potential for confusion between the similar-sounding ‘starboard!’ and ‘larboard!’ when shouted into the teeth of a gale. While the second half of larboard is ‘board’, the origin of the first part is not known. The drink port is a shortened form of Oporto in Portugal, from which the wine was shipped. See also porter