臀部，屁股和腚： When to use these and other words for buttocks? There are a number of words for buttocks in mandarin (including those in the title). In which situations should they be used?  Which are medical terms, colloquial terms, etc.?

If you wanted to say to a doctor "my buttocks are hurting" which would you use?
If you wanted to say to a friend "that guy/gal has a sexy butt" which would you use?
If you wanted to say to a partner "the kids bum is smelling, change their nappy" which would you use?

Thanks!
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.