Man A and Man B are talking: Man A: I like your wife. I think she is very beautiful. In fact, I would like to take her to a hotel tonight. Man B: You can't be serious!

Or we can paraphrase this cetence in another way,that's "You must be joking!"

You don't know what this mean? You can't be serious. lol (I'm kidding. :P)

"Earth is a disc and if you sail to wide into the ocean you will fall down off its rim" - "You can't be serious" or "In the first session of the new elected parliament the members of parliament decided to decrease their own monthly salary by 15 per cent to show their commitment to the fight against the increasing poverty in their country." - chorus: "You can't be serious"