Top: buttock from the side 尸, rest: come, go out 出 (Two mountains 山 coming out one after the other)
Mnemonic
("plumber's crack or smile", refers to the visible butt-crack if sb. with a too short shirt crouches) The buttock (=the crack) comes out when stooping (=bowing).
Think to the "carpenter crack", which refers to the visible butt-crack if a guy with a too short shirt crouches. The mnemonic: "The butt comes out, when crouching.
The radical shikabane has the meaning: corpse 尸. But because of its use in tail 尾, buttocks 尻 or urine 尿, it is generally considered to mean "buttocks".
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