Top: cave/hole 穴 (It is covered 宀 and a narrowing path 八 leads into the cave), below: stoop/crouch 屈 (The buttock 尸 [=butt's crack] comes out 出 when stooping: = "plumber's crack or smile")
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".
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.
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