The courts actually have a definition for that, though it's kind of loosey goosey... (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. All 3 conditions must apply to be considered by the courts to be legally obscene. I wish I could go back in time and fix the Ardennes Offensive.