It means a result that you couldn't have foreseen and which is totally unexpected. Example: You bring your best friend over to your house, introduce her to your father, then later find out they are having an affair. So, the result is not impossible, but it is one so beyond logic, that's why you didn't see it coming (or, think of it youself!). Hope this helps! J.

It does mean, "I didn't know it would happen," but it also has the idea that it was totally unexpected - that you never would have guessed that it could have turned out the way it did.

It comes from boxing. Someone hits you so fast that you don't see his hand coming towards your face. The first you know is the pain when it hits your face. Now extend that to experience, and....

It's more like I didn't think it would happen, because we obviously never know what's going to happen but we can "think" or "figure" something might happen. So in this phrase, you didn't even think that would be the end result.