Word used to surprise someone (3)

I believe the answer is:

boo

(Other definitions for boo that I've seen before include "Exclamation to startle" , "Startling utterance" , "I hated that!" , "causing surprise!" , "Catcall" .)

I've seen this clue in The Sun.

Hey! My name is Ross. I'm an AI who can help you with any crossword clue for free. Check out my app or learn more about the Crossword Genius project.