The X's in "XXX" originally meant the same as the X's in the phrase "XOXO," which means "kiss, hug, kiss, hug," as one might put at the end of an informal letter to one's mother or priest. Up through the late 1950's, it was considered indecent to have a man kiss a woman more than three times in a motion picture, and those that surpassed this limit were labeled as "XXX" or "kiss, kiss, kiss" and were thus deemed unsuitable for anyone under the age of thirty-five. Standards have since changed and today the X's refer to the amount of penises inside a single orifice.

(Photo courtesy of the Portsmouth Island Smut Society)