knock someone flat

(verb) to amaze someone, to surprise someone

Example Sentences:

Girlfriend: I have some news that will knock you flat.
Boyfriend: What?
Girlfriend: I’m pregnant.
Boyfriend: Oh.

Matthew’s parents were knocked flat when he told them he was joining the army.

The magician was knocking the audience flat with his impressive magic tricks.

Media:

Similar idioms:

1. knock one’s socks off
2. blow one’s mind
3. surprise, surprise