shirizumo; back-to-back sumo; buttocks pushing game
A traditional Japanese game where two players sit back-to-back and try to push each other out of a ring using only their buttocks. Often played by children.
We played shirizumo at the sports day.
尻相撲は意外と難しい。
Shirizumo is surprisingly difficult.
Compound of 尻 (shiri, 'buttocks') and 相撲 (sumō, 'sumo wrestling'). The exact origin is uncertain, but it is a traditional children's game.