Top: buttock from the side 尸, below: repeat, again 復 (to go 彳 with a reverse 复) [reverse = Who first lies on the tatami mat 日 and then sits cross-legged 夂, has "reversed"]
Mnemonic
For the buttock it is a repetition when shoes are put on.
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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