said suddenly to surprise someone who is unaware of one's presence
‘Boo!’ she cried, jumping up to frighten him
said to show disapproval or contempt
‘There's only one bar.’ ‘Boo!’
say ‘boo’ to show disapproval of a speaker or performer
they booed and hissed when he stepped on stage
the team were booed off the pitch
a person's boyfriend or girlfriend
got a call this morning from my boo
he swings by to surprise his boo with some flowers