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