Frightening word that's repeated in error (3)
I believe the answer is:
boo
'frightening word' is the definition.
(word that may be said to surprise someone)
I don't understand the remainder of the clue.
Can you help me to learn more?
(Other definitions for boo that I've seen before include "Voice disapproval" , "Express noisy disapproval" , "Shout in disgust" , "Exclamation to startle" , "Shout of dislike" .)