A loud sound that people make with their mouths closed.
The word 'bo' can be used to surprise or scare someone.
Examples:
I heard a loud 'bo' in the dark.
The 'bo' scared me!
He made a 'bo' sound to surprise his friend.
The 'bo' came from the closet!
The 'bo' was so loud, it shook the house!
I like to make a 'bo' sound to scare my sister.