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]

WordNet (r) 3.0 (2006):

B.O.
    n 1: malodorousness resulting from a failure to bathe [syn:
         body odor, body odour, B.O.]