not know what hit (one)
To be extremely shocked, surprised, or confused by something. When my father passed away, I just didn't know what hit me. With this new strategy, our opponents won't know what hit them.
not know what hit you be hit, killed, or attacked by someone or something without warning.
not know what ˈhit you (informal) be so surprised by something that you do not know how to react: You should have seen his reaction! He didn’t know what had hit him! OPPOSITE: see something coming