"harbor," Old English port "harbor, haven," reinforced by Old French port "harbor, port; mountain pass;" Old English and Old French words both from Latin portus "port, harbor," originally "entrance, passage," figuratively "place of refuge, asylum," from PIE *prtu- "a going, a passage," suffixed form of root *per- (2) "to lead, pass over."
Meaning "left side of a ship" (looking forward from the stern) is attested from 1540s, from notion of "the side facing the harbor" (when a ship is docked). It replaced larboard in common usage to avoid confusion with starboard; officially so by Admiralty order of 1844 and U.S. Navy Department notice of 1846. Figurative sense "place of refuge" is attested from early 15c.; phrase any port in a storm first recorded 1749. A port of call (1810) is one paid a scheduled visit by a ship.