臀部，屁股和腚： When to use these and other words for buttocks?
Interesting question.


屁股: the most common way to say butt. Sometimes may be thought as rude or uneducated when using in public. If you are going to a doctor, this is the word to go.
臀部: (may be the only) elegant way of saying butt. The most formal way in written Chinese. When used in oral language in recent days, usually it's only used on woman, and sometimes people may think this word having sexual intimation.
腚: used in some Chinese accents.
尻: used in some Chinese accents and ancient literature.