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").

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Definitions of aboard from WordNet

aboard (adv.)
on a ship, train, plane or other vehicle;
Synonyms: on board
aboard (adv.)
on first or second or third base;
Their second homer with Bob Allison aboard
aboard (adv.)
side by side;
anchored close aboard another ship
Synonyms: alongside
aboard (adv.)
part of a group;
Bill's been aboard for three years now