flap seats

Noun

flap seat (plural flap seats)

1. A hinged seat that can be raised when not in use.
   • And be it enacted that […] no person […] presume to wish to go out for ginger-beer or oysters between the acts, to the annoyance of the nine rows of audience behind him, and the especial discomfiture of the fat old gentleman with the child on his knees, who sits on the flap-seat by the box-door.1843, Albert Richard Smith, “The Bureau Drawer”, in The Wassail-Bowl, volume 1, London: Richard Bentley, pages 115–116

   Synonyms:

   folding seat, tip-up seat

2. A flap on the part of an undergarment or night garment that covers the buttocks.
   • After several minutes, Liebknecht stood barefoot in a suit of long winter underwear. Some of its buttons were missing and the flap seat was baggy from too much laundering.1954, Ben Hecht, A Child of the Century, New York: Simon and Schuster, Book 4, p. 273