not know what hit you
US /nɑt noʊ wʌt hɪt ju/
UK /nɑt noʊ wʌt hɪt ju/
1.
to be completely surprised or overwhelmed by something, often something negative or sudden
:
•
When the market crashed, many investors didn't know what hit them.
•
The boxer landed a punch so fast, his opponent didn't know what hit him.