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]

Moby Thesaurus II by Grady Ward, 1.0:

21 Moby Thesaurus words for "boo":
   Bronx cheer, Mary Jane, bird, booing, cannabis, catcall,
   give the bird, give the raspberry, grass, hiss, hissing, hoot,
   pooh, pooh-pooh, pot, raspberry, razz, tea, the bird, weed,
   whistle at