"piece of timber sawn flat and thin, longer than it is wide, wider than it is thick, narrower than a plank;" Old English bord "a plank, flat surface," from Proto-Germanic *burdam (source also of Old Norse borð "plank," Dutch bord "board," Gothic fotu-baurd "foot-stool," German Brett "plank"), perhaps from a PIE verb meaning "to cut." See also board (n.2), with which this is so confused as practically to form one word (if indeed they were not the same word all along).
In late Old English or early Middle English the sense was extended to include "table;" hence the transferred meaning "food" (early 14c.), as "that which is served upon a table," especially "daily meals provided at a place of lodging" (late 14c.). Compare boarder, boarding, and Old Norse borð, which also had a secondary sense of "table" and an extended sense "maintenance at table." Hence also above board "honest, open" (1610s; compare modern under the table "dishonest"). A further extension is to "table where council is held" (1570s), then transferred to "leadership council, persons having the management of some public or private concern" (1610s), as in board of directors (1712).
"Bow to the board," said Bumble. Oliver brushed away two or three tears that were lingering in his eyes; and seeing no board but the table, fortunately bowed to that.
Meaning "table upon which public notices are written" is from mid-14c. Meaning "table upon which a game is played" is from late 14c. Meaning "thick, stiff paper" is from 1530s. Boards "stage of a theater" is from 1768.
"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.
Meaning "to be supplied with food and lodging" (from n.1 in transferred sense) is from 1550s. Transitive meaning "provide with daily meals and lodging" is from 1590s. Related: Boarded; boarding.