I agree with you on this point. If we consult a dictionary when we see someone dancing on the stage and try to say to the person "お尻振って～!! in English", we can find the English equivalent of "尻" is "buttock" or "hip". So most of us Japanese who are studying English may say to the dancing person "Shake your buttock!" or "Shake your hip!"
However, maybe these expressions are unnatural, aren't they? Maybe we should say "Shake your butt!" or "Shake your hips!"
But I have not heard someone saying "Shake your butt!" or "Shake your hips!". I have heard something like this. "Shake your booty!!" So I feel that the expression "Shake your booty!!" is more natural than the others.
By the way, even if we consult a dictionary for the word "尻", it is difficult for us to find out the English "booty". When we search the English equivalent of "尻", we can only get the English words such as buttock, hip, bottom, behind, rear end, rump, seat, something like that.
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