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Main Entry: ↑hit
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know what hit you
— used in negative statements to say that something you did not expect surprised you very much
I didn't know what hit me–suddenly I just felt so dizzy.
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]
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Main Entry: ↑know
Useful english dictionary. 2012.