[sit-uh-pon, -pawn]
  1. a piece of waterproof fabric or other material carried by campers, hikers, etc., and used for sitting on wet surfaces.
  2. Chiefly British Informal. the buttocks.

Origin of sit-upon

First recorded in 1835–45; noun use of verb phrase sit upon Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2018