Hi Bean It's slightly old fashioned and means something like 'please tell me right now', or 'come on, spill the beans!' example - "I have a little secret, would you like to hear it?" "Oh yes, yes, do tell!" In speech it is usually said in a slightly excited way, or even with a conspiratorial tone.

I think it means: It's hard to believe! /I can't believe it! (...what you tell me.)