Top: var. of buttock seen from the side 尸, below: opening 口 as anus, in the sense of 'behind' (Originates in the custom that the empress followed behind the emperor.)
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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