I'm not sure I should answer this question here. You might try yahoo answers. But anyway: When the movie rating system was first created, they used "X" for movies that were too violent or sexual. Directors complained that porno movies that aired in public theaters at the time, and movies that did not have actual sex in them, were receiving the same "X" rating. So, two new ratings were created - R, and NC-17 and the old "X" rating was abandoned. Because of porn movies at the time which aired in public theaters, whenever the public heard the rating "X" they began to think about porn. So, the porn industry has continued to use the "X" rating, but in order to make more sales, they used "XXX" = meaning really, really raunchy or sexual. "XXX" also makes people think of "censored" which makes people want to see the movie.. at least that's what the movie makers hoped.

Three letters might be the clue. Is this really so hard?