boo¹

buː/

exclamation
exclamation: boo

1.
said suddenly to surprise someone who is unaware of one's presence.

"‘Boo!’ she cried, jumping up to frighten him"

2.

said to show disapproval or contempt.

"‘There's only one bar.’ ‘Boo!’"

noun: boo; plural noun: boos

1.
an utterance of ‘boo’ to show disapproval of a speaker or performer.

"the audience greeted this comment with boos and hisses"

verb: boo; 3rd person present: boos; past tense: booed; past participle: booed; gerund or present participle: booing

1.
say ‘boo’ to show disapproval of a speaker or performer.

"they booed and hissed when he stepped on stage"

Origin
early 19th century (in sense 2 of the exclamation): imitative of the lowing of oxen.

boo²

buː/

noun informal•US
noun: boo; plural noun: boos

a person's boyfriend or girlfriend.

See Also:

Boo the ghosts from the Super Mario series.

King Boo, the leader of the Boos, the main antagonist of the Luigi's Mansion series, and the arch-enemy of Luigi.