It comes from the Supreme Court's 5-4 ruling in Miller v. California (1973), in which Chief Justice Warren Burger (writing for the majority) held that obscene material is not protected by the First Amendment. The definition he used went like this:
The basic guidelines for the trier of fact must be: (a) whether "the average person, applying contemporary community standards" would find that the work, taken as a whole, appeals to the prurient interest ... (b) whether the work depicts or describes, in a patently offensive way, sexual conduct specifically defined by the applicable state law, and (c) whether the work, taken as a whole, lacks serious literary, artistic, political, or scientific value. If a state obscenity law is thus limited, First Amendment values are adequately protected by ultimate independent appellate review of constitutional claims when necessary.Or, to put it in much more casual (and obviously non-legal) terms:
- Is it porn?
- Does it actually show sex?
- Is it otherwise useless?