boo

from 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}]

from 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]

from 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]

from 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

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