(a) bolt from the blue

To refer to something as a bolt from the blue means that it happened totally unexpectedly and was a complete surprise.
“The chairman’s resignation came as a bolt from the blue.”

out of the blue

If something happens out of the blue, it happens unexpectedly and causes a surprise.
“I had nearly given up hope when out of the blue I was offered a job”

caught unawares

If someone is caught unawares, they are surprised and unprepared for what happens.
“The security guard moved so silently that the thief was caught unawares.”

credibility gap

The extent of disbelief, of the difference between what you are asked to believe and what you are able to believe, is called a credibility gap
“The growing credibility gap may lead to a serious loss of votes in the next elections.”

do a double take

Someone who does a double take looks again in surprise at something unexpected.
“He did a double take when he saw his wife in a restaurant with another man.”

drop a bombshell

If you drop a bombshell, you make an unexpected announcement which will greatly change a situation.
“The chairman dropped a bombshell when he announced the merger with the company’s biggest rival.”

eyes on stalks

If your eyes are on stalks when you look at something, they are wide open with surprise or amazement.
“The child’s eyes were on stalks as he watched the magician’s performance.”

(your) jaw drops

If someone’s jaw drops, they show total amazement.
“When the prize was announced, the winner’s jaw dropped.”

jump out of one’s skin

If you jump out of your skin, you are extremely surprised or shocked.
“Jane nearly jumped out of her skin when the horse put its head through the kitchen window!”

knock your socks off

If something amazes you, or impresses you greatly, it knocks your socks off.
“The magnitude of the project will knock the socks off everyone in the office.”

knock down with a feather

To say ‘you could have knocked me down with a feather’ emphasizes the fact that you were extremely surprised.
“When I heard the name of the winner, you could have knocked me down with a feather!”

lo and behold!

The term ‘lo and behold’ is used to express surprise, especially at a sudden or unexpected appearance.
“I was watering the flowers when, lo and behold, there was the watch I’d lost”

(a) nine-day wonder

An event which isa nine-day wonder causes interest, surprise or excitement for a short time, but it doesn’t last.
“His sudden departure was a nine-day wonder but he was soon forgotten.”

raise eyebrows

Someone who raises their eyebrows at something shows surprise or disapproval by the expression on their face.
“When the boss arrived in jeans, there were a lot of raised eyebrows

rooted to the spot

If you are so shocked, surprised or scared that you are rooted to the spot, you reaction is so strong that you are unable to move.
“Joe stood rooted to the spot as the plane landed on the water.”

seeing is believing

The expression ‘seeing is believing’ means that when you actually see something that seems incredible you can be sure it exists, or that what you have been told is really true.
“Mark says bananas grow in his garden, but seeing is believing!”

stop dead in tracks

If you stop dead in your tracks, you stop suddenly because you are totally surprised or frightened.
“When Steve saw the snake, he stopped dead in his tracks.”

struck dumb

If someone is struck dumb, they are unable to speak because they are so surprised, shocked or frightened by something.
“The accused man was struck dumb when the verdict was announced”

taken unawares

If something takes you unawares, it surprises you because you were not expecting it.
“The driver’s angry reaction took me unawares.”

there’s no accounting for taste

This expression is used to indicate surprise at another person’s likes or dislikes.
“She fell in love with a guy who’s short, fat, bald and poor … there’s no accounting for taste!”

wonders will never cease

The expression ‘wonders will never cease’ is used to express pleasure or surprise at something.
“The price of petrol has dropped! Wonders will never cease!”

words fail me

The expression ‘words fail me’ is often used when someone is so shocked, surprised or touched by something that they don’t know what to say.
“What do you think of Bob’s attitude?” “Words fail me!