Stock phrase when a character has heard of something that crosses a significant line of comfort for him. It may be something that he thought couldn't happen... or something he thought no one would be despicable enough to do. In any case, if a character says this, he has been shocked out of his comfort zone. Of course, it's often used parodically, when the "sacred" thing is of relatively little consequence: "And the new boss has cancelled 'Casual Fridays'." "Is nothing sacred?!" Its also Rule 42 of the internet. That is, that no, nothing is. Compare There Should Be a Law.