Source : Webster's Revised Unabridged Dictionary (1913)

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}.]

Source : WordNet®

boo
     v : show displeasure, as after a performance or speech [syn: {hiss}]
         [ant: {applaud}]

boo
     n : a cry or noise made to express displeasure or contempt [syn:
          {hoot}, {Bronx cheer}, {hiss}, {raspberry}, {razzing}, {snort},
          {bird}]