Slang - colloquial expression used when referring to the buttocks, however can be used to additionally refer to breasts. When using this term, keep in mind that plurality is almost always implied so the entire set of cheeks or breasts are referenced by this term.
Examples
"Please stop looking at my buns.
"Do these pants make my buns stick out?"
"I've heard that Jessicapierce has a mythical set of buns."
"Hot cross buns"