BO (also B.O.) is an abbreviation for body odor, and refers mainly to objectionable odors from the armpits.

When the film channel HBO debuted on cable TV, kids asked each other "Do you have HBO?", and if the answer was yes, gleefully screamed "You've got horrible body odor and you admit it!"

The letters BO are also the initials of yours truly, which is why I often use my full initials, BAO.