1795 W. Felton II. (Gloss.) 218 Drop Bottom, the bottom of a coach, chariot, or chaise body when sunk deeper than the surface of the framing, to give room.
1835 A. Ure 44 It raises the coals..and delivers them on an elevated railway platform into a waggon—through the drop-bottom of which they are duly distributed among the range of hoppers attached to Stanley's ingenious furnace-feeding machines.