Nearly everyone has seen the 1987 movie The Princess Bride (and if you haven’t, you should), and knows that "inconceivable" is the favorite word of Vizzini (played by Wallace Shawn).
Eventually, Inigo Montoya (Mandy Patinkin) calls him on it.
Well, actually, Inigo, it does. Other ways of saying it are "impossible," "out of the question," and "unimaginable." Those would probably better than "inconceivable," given the circumstances.
They wouldn’t be as funny, though, which is what they were going for here.