This radical is considered to be a simplification of: fortune telling 占 (crack 卜 in the shell 口 of a turtle from which the fortune was told.). Sometimes it is read as a forking of a road.
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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