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c. on board: on one side, close alongside (of a ship or shore); also as prep., short for on board of. (See also 14) to lay (a ship) on board: to place one's own ship alongside of (it) for the purpose of fighting. to run on board (of), to fall on board (of): literal to run against, fall foul of (a ship); figurative to make an attack, fall, upon (a person or thing). on even board with: exactly alongside with; figurative on even terms with, ‘square’ with.

a. on board (beside the technical sense in sense 12c) has now, in common use, the meaning: On or in a ship, boat, etc.; into or on to a ship. That this expression is elliptical, is witnessed by the fuller form on ship-board (cf. Middle English ‘within schippe burdez’ in sense 12), and the construction ‘on board of the ship’, or ‘on board the ship’ (where it is perhaps often supposed that ‘board’ means the deck). Hence board-ship used attributively or as an adjective.On board appears to be a later expansion (cf. afoot adv., on foot at foot n. and int. Phrases 3a) of aboard adv. and prep., a-bord, and this to have been taken directly from French à bord, as in aller ou monter à bord, être à bord, short for au bord du vaisseau, in which bord ‘ship's side’ comes contextually to be equal to ‘ship’ itself. Similar phrases are used in other modern Germanic languages, as Dutch aan boord, German an bord, Swedish, Danish om skibsbord. Although on borde occurs poetically in Old English, and vpon borde in Middle English, in sense of ‘in, upon ship’, these appear to have no historical connection with the later a-board, which begins about 1500, and on board, which appears late in the 17th cent.

b. on board is used as preposition for on board of.