This phrase was first used by U.S. Supreme Court Justice Potter Stewart, in a case of obscenity. Instead of defining it—we would think that this would be a requirement for a law, which is of course backed by threat of imprisonment—he resorted to what might be called Begging Common Sense. It’s just common sense, it’s easy-peasy, there’s no need to define the term…
This is not a satisfactory approach. Leaving aside concerns with undefined terms as the basis of sending someone to jail, it is an admission that one cannot define the term. As Richard Feynman once said:
“I couldn’t reduce it to the freshman level. That means we really don’t understand it.”
Obscenity has an analog in monetary economics: usury. Most people think usury is a bad thing, like obscenity. But they can’t define it.