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.
"left-hand side of a ship" (to a person on board and facing the bow), 1580s, alteration of Middle English ladde-borde (c. 1300), perhaps literally "the loading side," if this was the side on which goods were loaded onto a ship, from laden "to load" (see lade) + bord "ship's side" (see board (n.2)).
Altered 16c. by influence of starboard, then, to avoid confusion of similar-sounding words, it was largely replaced by the specialized sense of port (n.4). The Old English term for it was bæcbord, literally "back board" (see starboard), a term which remains in the other Germanic tongues.
"the left side of a ship" (looking forward from the stern), 1540s, probably from the notion of "the side facing the harbor" (when a ship is docked); thus from port (n.1). On old-style vessels the steering oar was on the right side, thus they would tie up at a wharf on the other side. 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. As an adjective by 1857.
U. S. Navy Department, Washington, Feb. 18, 1846.
It having been repeatedly represented to the Department that confusion arises from the use of the words "larboard" and "starboard"' in consequence of their similarity of sound, the word "port" is hereafter to be substituted for "larboard." George Bancroft, Sec. of the Navy.
The whalemen are the only class of seamen who have not adopted the term port instead of larboard, except in working ship. The larboard boat was this boat to their great-grandfathers, and it is so with the present generation. More especially is this the case in the Atlantic and South Pacific fleets; but recently the term port-boat has come into use in the Arctic fleet. [Fisheries of U.S., V. ii. 243, 1887]
"side of ship," Old English bord "border, rim, ship's side," from Proto-Germanic *burdan (source also of Old Frisian bord, Old Saxon bord, Dutch boord "border, edge, ship's side," German Bord "margin, border," Old High German bart, Old Norse barð "margin, shore, ship-board"), perhaps from the same source as board (n.1), but not all sources accept this. Connected to border; see also starboard.
If not etymologically related to board (n.1), the two forms represented in English by these words were nonetheless confused at an early date in most Germanic languages, a situation made worse in English because this Germanic word also was adopted in Medieval Latin as bordus (source of Italian and Spanish bordo) and entered Old French as bort "beam, board, plank; side of a ship" (12c., Modern French bord), via either Medieval Latin or Frankish, and from thence it came over with the Normans to mingle with its native cousins. By now the senses are inextricably tangled. Some etymology dictionaries treat them as having been the same word all along.
To go by the board originally was "fall overboard" (1757), of a mast, etc., hence, generally, "be completely lost or destroyed" (1835). To be on board is from c. 1500, originally nautical, "close alongside;" then, less technically, "on the ship" (1708), perhaps by influence of aboard, or from the noun in the sense "plank;" extended to trains, planes, general situations.
by 1914 (1903 as push), a word of uncertain origin, but there is no evidence for the common derivation from an acronym of port outward, starboard home, supposedly the shipboard accommodations of wealthy British traveling to India on the P & O Lines (to keep their cabins out of the sun); as per OED, see objections outlined in G. Chowdharay-Best in Mariner's Mirror, January 1971; also see here. The acronym story dates from 1955.
More likely it is from slang posh "a dandy" (1890), from thieves' slang meaning "money" (1830), originally "coin of small value, halfpenny," possibly from Romany posh "half" [Barnhart].
The cavalryman, far more than the infantryman, makes a point of wearing "posh" clothing on every possible occasion — "posh" being a term used to designate superior clothing, or articles of attire other than those issued by and strictly conforming to the regulations. [E. Charles Vivian, "The British Army From Within," London, 1914]