From WordNet (r) 2.0 (August 2003) [wn]:
boo
n : a cry or noise made to express displeasure or contempt [syn:
hoot, Bronx cheer, hiss, raspberry, razzing,
snort, bird]
v : show displeasure, as after a performance or speech [syn:
hiss] [ant: applaud]
From The Collaborative International Dictionary of English v.0.48 [gcide]:
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]
From The Collaborative International Dictionary of English v.0.48 [gcide]:
boo \boo\ v.
to show displeasure (after a performance or speech) by making
a prolonged sound of "boo".
[WordNet 1.5]