The 'XXX' has rather mundane origins, as it is simply a marketing attempt made by a movie studio that used an already existing 'X' rating that was introduced by MPAA in 1968 to mark the movies that were considered unsuitable to minors, possibly based on the likewise named rating introduced in 1951 in United Kingdom (to be precise, it meant that customers under 16 were admitted under no circumstances). The first movie getting this rating was 'Vixen!' that premiered on 22nd October 1968 (some sources give this credit to a French movie 'The Girl on a Motorcycle', although it premiered in USA a month later).

There was also one important difference between the 'X Rating' and three others (G, M and R) was that the former was not trademarked, meaning that anyone could market their movies as 'X-Rated', causing the rating differences between the high-quality movies, such as Midnight Cowboy or Last Tango in Paris and cheap pornographic flics to blur, which issue has been discussed by the National Association of Theatre Operators with MPAA as early as 1970. As erotic and pornographic movies were by definition not suitable for minors, and erotica was subjected to much stricter censorship in comparison to e.g. violence in Unites States, the 'X Rating' became generally associated with sexual content. This made the pornographic industry and 'X Rating' synonymous to some extent, especially since the late 1970s when many mainstream movies containing sex and violence scenes were either edited or toned down to secure the 'R' rating (also partially to avoid the aforementioned association with pornography).

Of course, the pornographic film companies were all too eager to emphasize that their products are aimed on adult audience only, and thus started to multiply the 'X', suggesting even more graphic content as a way to attract customers. The first movie to be advertised this way was the 1969 picture Starlet, a poster for which contained the 'XXX' followed by a slogan So adult, one 'X' is not enough! which idea was quickly picked up bu others. Although one can sometimes find the explanation that the number of 'X' letters meant the type of the movie, with 'X' being erotica, 'XX' being graphically sexual and 'XXX' showing hardcore sexual scenes, this was never codified in any way by the pornographic industry and MPAA used only the single 'X'. In 1990, MPAA finally cancelled the 'X' rating replacing it with 'NC-17', cementing the association of 'X' with cinematic erotica and pornography.

Thus, the modern usage of 'XXX' hails back to an advertisement of a 1969 movie, where it has been used as a pun referring to a recently introduced MPAA rating system.

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Bisen, C.S., Film Censorship: Regulating America's Screen, Wallflower Press, New York 2018