This phrase literally means , 'Well what do you know?' it is an American idiomatic expression of surprise and tends to mean, 'Well fancy that, I didn't know that fact'. For example you may say to a friend, 'Guess what? Minako is getting married soon'. Your friend Tomato may be very surprised and she'll reply to you,'Really? Well whaddya know! ' Australians tend to say,' well, well, well' or' you could have knocked me over with a feather!' or 'fancy that!'