Entries linking to onboard
"in a position above and in contact with; in such a position as to be supported by;" also noting the goal to which some action is or has been directed; "about, concerning, regarding; in a position to cover;" as an adverb, "in or into a position in contact with and supported by the top or upper part of something; in or into place; in place for use or action; into movement or action; in operation," Old English on, unstressed variant of an "in, on, into," from Proto-Germanic *ana "on" (source also of Dutch aan, German an, Gothic ana "on, upon"), from PIE root *an- (1) "on" (source also of Avestan ana "on," Greek ana "on, upon," Latin an-, Old Church Slavonic na, Lithuanian nuo "down from").
Also used in Old English in many places where we now would use in. From 16c.-18c. (and still in northern England dialect) often reduced to o'. Phrase on to "aware" is from 1877.
"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.