Is it farther or further?
late 14c., probably in most cases from Old French à bord, from à "on" + bord "board," from Frankish *bord or a similar Germanic source (see board (n.2)); the "boarding" or sides of a vessel extended to the ship itself. The usual Middle English expression was within shippes borde. The call all aboard! as a warning to passengers is attested from 1838.