The Collaborative International Dictionary of English v.0.48:

Bo \Bo\, interj. [Cf. W. bw, an interj. of threatening or
   frightening; n., terror, fear, dread.]
   An exclamation used to startle or frighten. [Spelt also boh
   and boo.]
   [1913 Webster]

Moby Thesaurus II by Grady Ward, 1.0:

39 Moby Thesaurus words for "BO":
   bad breath, bad smell, beaded brow, beads of sweat, body odor,
   cold sweat, diaphoresis, exudate, exudation, fetidity, fetidness,
   fetor, foul breath, foul odor, frowst, graveolence, halitosis,
   honest sweat, lather, malodor, mephitis, miasma, nidor,
   noxious stench, offensive odor, perspiration, perspiration odor,
   reek, reeking, rotten smell, stench, stench of decay, stink,
   streams of sweat, sudor, sudoresis, sweat, swelter, water



V.E.R.A. -- Virtual Entity of Relevant Acronyms (February 2016):

BO
       Back Orifice (CDC)