Off the hook!

I came across a phrase while reading and it goes something like this, "hit somebody with a left hook". I think it is an American phrase. Can someone enlighten me on its meaning?

A 'hook' is a fighting term for a blow that is swung round the side - i.e. is shaped like a hook. I think it predates American-English due to simplicity.

the left or right is the arm that delivers the blow.