Top: buttocks seen from the side 尸, below: old 古 (originally derived from the image of a skull, hence "old"; the idea of a burial mound is also helpful)
Mnemonic
Where the buttocks becomes old, he has his residence.
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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