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]

The Collaborative International Dictionary of English v.0.48:

boo \boo\ v.
   to show displeasure (after a performance or speech) by making
   a prolonged sound of "boo".
   [WordNet 1.5]

WordNet (r) 3.0 (2006):

boo
    n 1: a cry or noise made to express displeasure or contempt
         [syn: boo, hoot, Bronx cheer, hiss, raspberry,
         razzing, razz, snort, bird]
    v 1: show displeasure, as after a performance or speech [syn:
         boo, hiss] [ant: acclaim, applaud, clap, spat]