noun
plural boos
[count] :a sound that people make to show they do not like or approve of someone or something
[noncount] informal :any sound at all - usually used in negative statements
A person who (US) wouldn't say boo or (Brit) wouldn't say boo to a goose is very quiet and shy.
interjection
used to show dislike or disapproval of someone or something
When people say Boo! with this meaning, they say it very slowly.
used when a person frightens someone
When people say Boo! with this meaning, they say it very quickly.
verb
boos; booed; booing
to show dislike or disapproval of someone or something by shouting Boo slowly [no obj]