boo

Concise Oxford English Dictionary © 2008 Oxford University Press:
boo/buː/

▶exclamation

• 1 said suddenly to surprise someone.

• 2 said to show disapproval or contempt.

▶verb (boos, booing, booed) say ‘boo’ to show disapproval or contempt.

– phrases
wouldn't say boo to a goose used to emphasize that someone is very shy or reticent.

– origin C19: imitative of the lowing of oxen.