noun
mythical anal ball sought by kappa
In Japanese folklore, a mythical ball believed to reside inside the anus; kappa are said to try to extract it, often drowning victims in the process. The term is primarily encountered in folk tales and discussions of yōkai.
See also: 河童
河童は人間の尻子玉を抜こうとするという話がある。
There is a story that kappa try to pull out the shirikodama from humans.
河童 is the mythical creature that seeks the 尻子玉, while 尻子玉 is the object itself.
Compound of 尻 (shiri, 'buttocks') + 子 (ko, diminutive or small thing) + 玉 (tama, 'ball'). The exact origin of the folk belief is uncertain, but it is a well-known element of kappa lore.