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What are some idioms that no longer make sense? Can you think of a modern replacement?
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Arse over tit is in an identical British expression for the same thing
https://en.wiktionary.org/wiki/ass_over_teakettle
It's the same as 'head over heels' in that the bottom part of your body (your ass) and the top part of your body (your breast/chest) switch places.
I feel like we have to assume the description of how your body moves is the prime definition (head over heels, or any variation) and any substitution of words like ass over teakettle is a playful alteration which works and is understood primarily because people are familiar with the original and understand the alteration through context of use.