This is about the contemporary1 meaning of "ass" in the US and other places.
Have you ever noticed how things related to sex are considered "dirty" by many, which makes people guiltily reluctant to call a spade a spade, and that this leads to the creation of a plethora of synonyms? Don't take my word for it, look at Velocity's list of synonyms for masturbation! Granted, that word just seems to have parameters of sexiness, guilt and sneaky underhandedness (pun only partly intended) that make it a winner in terms of synonym word list size. But you get the idea.
One of the runners-up in this category is the word for the nates2. Here's the list of synonyms I'm aware of:
Just as a curiosity: one German synonym, used especially with children, is "Popo". I think this sounds kind of cute. People who consider themselves too dainty to utter this word may say things like "Setz dich auf deine vier Buchstaben!" – "Sit on your four letters!". In more adult circles, "Popo" is often abbreviated to "Po".
Another anal linguist tells me that: "in Mandarin Chinese, ass is 'pigu'; pi = bottom or fart and gu = groin area. It's pronounced 'pee-gwu'."
That's all I can think of, off the top of my head. If you know of more, I urge you to /msg me so I can extend the list.
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Footnotes
- 9 out of 10 men who have tried riding donkeys... prefer women.
- Out of the list, the most objective and scientific-sounding synonym I have come across
- as in, "Baby's got back!"
- Gay motto: "Never leave your friends behind!"
- as in, "Shake your booty!"
- British usage
- North American usage