(noun) the buttocks, or in some cases, the butthole. It is made to sound like the spanish equivalent to buns, but in fact is not spanish at all.
Boss: "Have you finished those reports yet!?!
Guy: "How about you shove it up your buniertos!!"
Girl on subway: "AHH! Someone just grabbed my buniertos!!"
Girl: "Who are you going to vote for mayor?"
Guy: "I don't give a rat's buniertos who wins"