seat lining; lining of pants seat
Refers to the inner fabric panel sewn into the seat area of trousers or traditional Japanese hakama for reinforcement or comfort.
このズボンは尻当てが付いているので、長く履けます。
These trousers have a seat lining, so they last a long time.
Compound of 尻 (shiri, 'buttocks, seat') and 当て (ate, 'patch, lining').