.. a false “rump”, almost always some stadium cushion like pillow over the rear end. While the earlier 18th century “farthingale” (think bicycle rim around your waist) was metal and structured, it was not nearly the engineering accomplishment of the many types of early bustles in the first bustle era leading to 1880.
(Photos: modern reproduction of a 1780’s false rump vs. extant 1880-81 metal and cotton tape bustle).