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'.
When you are coming out 丷 of a Buddhist temple 开 you shall be united. - [开 = image of a temple's entrance gate with double upper traverses, which actually means "open", as these gates have no door]
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