|Justice Potter Stewart|
Anyone who has ever studied the First Amendment has probably read the phrase in conjunction with the decision rendered in Jacobellis v. Ohio, 378 U.S. 184 (1964). Jacobellis was a theater manager and was convicted of airing the french film The Lovers at his theater. The state claimed the film was obscene. Of course, Jacobellis took the state to court. He won the case and Stewart became, shall we say, infamous for his concurrence. This statement is famous that it even has its own Wikipedia page.
Here is the full statement
I shall not today attempt further to define the kinds of material I understand to be embraced within that shorthand description ["hard-core pornography"], and perhaps I could never succeed in intelligibly doing so. But I know it when I see it, and the motion picture involved in this case is not that.