Cry for Statler and Waldorf (3)

I believe the answer is:

boo

(Other definitions for boo that I've seen before include "Frightening word (for a goose?)" , "Show displeasure orally" , "Sound expressing disapproval" , "Sound made to show disapproval or contempt" , "Surprising exclamation" .)

I've seen this clue in The Wall Street Journal.

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