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!’

an utterance of ‘boo’ to show disapproval of a speaker or performer

the audience greeted this comment with boos and hisses

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