— used in negative statements to say that something you did not expect surprised you very much用于否定句，表示未料到的事令人吃惊
I didn't know what hit me—suddenly I just felt so dizzy.不知道怎么搞的，我突然感到头晕得厉害。
The company came out with an entirely new line of products last year, and their competitors never knew what hit them. [=their competitors were not prepared and were completely surprised]去年这家公司推出一款全新产品，令竞争对手猝不及防。