The character for tall 大 shows a person 人 with outstretched arms 一. If you shift the lower strokes towards each other, you can get the: woman 女. The wider lower area can be understood as female hips?
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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