similar to a fupa (Fat Upper Pubic Area) or bif, (butt in front), from the waste down, you can't tell whether if a bi butt is coming or going. A true bi butt actually has two fully formed butt cheeks on each side, two in front, two in back. The front or rear plumber's crack is optional and generally not visible, since a bi butt's pants tend to be belted across mid stomach area or even higher in some cases.
Notre Dame Football coach Charlie Weis has a bi butt. Some older women are also bi butts or bi buttual.