aboard (adv., prep.)
late 14c., "at the side of a ship;" mid-15c., "onto or aboard a ship," probably in most cases from the Old French phrase à bord (compare Old French aborder "to board (a ship)"), from à "on" + bord "board," from Frankish *bord or a similar Germanic source (see board (n.2)). The word for the "boarding" or sides of a vessel being extended to the ship itself. The usual Middle English expression was within borde. The call all aboard! as a warning to passengers (on ships or railway cars) is attested from 1829 (compare French aller à bord "go aboard").