In our early life, especially in mixed company, we learn that certain things are best left unsaid in public. Not to be silenced, we often create phrases that are a substitute for what we actually mean to say but can't publicly state.
For example, we rarely say we we are going to use the the toilet room. Instead, we substitute a word or phrase that makes the action seem more polite. We are going to the ladies room, or the powder room.
I think the same discretion holds true for talking about our intimate body parts. We are comfortable announcing that, after eating, our stomach feels stuffed but are not so quick to openly declare that we have a hungry 'private' part.
