If you hoot the horn on a vehicle or if it hoots, it makes a loud noise on one note.

If you hoot, you make a loud high-pitched noise when you are laughing or showing disapproval.

When an owl hoots, it makes a sound like a long 'oo'.

If you say that someone or something is a hoot, you think they are very amusing.

If you say that you don'tgive a hoot or don't care two hoots about something, you are emphasizing that you do not care at all about it.