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according to wikipedia:
"In the past x's were used to indicate kisses. Three x's, like xxx, would mean 3 kisses ... Starting the 70s due to the illicit nature of the pornography industry at the time three x's - which here indicated three kisses on tip of the penis - was a way to label movies in a covert way to escape the notice of law enforcement let still inform the porn consumer about the nature of the content."
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