1n. a light, four-wheeled vehicle having a single seat resting on springs that are attached to a springy platform.
1871  (1888)  Scarcely had the horse struck the current when he was thrown on his side, and horse and buckboard, with Mr. McDougall standing on the buckboard seat, were carried down the river.
1960  At the hotel hitching-rail a team of horses stood dismally, the buckboard creased and dusted with snow.
2n. a plank placed across the box of a farm wagon and used as a passenger seat.
1887  ". . . I fixed it to go with a lot of young fellers in an old farm wagon with a sort of buckboard for a middle seat. This buckboard, Cap, was a two inch plank and hed no more spring in it than a brick."