The Etsy seller says:
This corset-like Body-enhancing hourglass shape enducing undergarment is attributed to a very famous New York Costume Design house/designer, Barbara Matera (Bio below). Barbara Matera designed all of the original costumes for La Cage Aux Folles, and I believe this is where this piece originates from.
This was clearly meant for a male actor playing female roles or masquerading as a woman, as it is enhanced with ample bust padding and hip padding and is boned. The original actor’s name(s)(Gary Slavin, Gary or Slavin) are written on the label inside. Alternately I believe it could have been used to give a thinner actress some shape on the stage.