Although similar to the DS, the SM was a more special car that was powered by a Maserati-built engine.
With its nickname "the goddess" and its curvaceous bodywork, the Citroen DS is one of the more sexualized cars there has ever been. But the Citroen SM took both the technology and the sexual undertones of the DS and pushed them just that little bit further. Being a GT car, rather than executive sedan, the SM was also just a more exciting car all around, and it remains one of the most sadly underappreciated cars in history. Though it came 15 years later and was an entirely different kind of car, the SM owed it existence to the DS.
It was as far back as 1961 that Citroen started looking into a sport version of the DS, which had initially debuted in 1955. Several different prototypes were built, but Citroen was reluctant to design and build a whole new engine for the car, and none of its existing engines really provided enough power. The project would end up being shelved until 1968, when Citroen bought Maserati. So the bosses at Citroen had Maserati build them a new engine just for the SM. The V6 which ended up going into the SM had dual overhead cams and a 90-degree V, which some people took to mean that the engine was essentially just Maserati's own V8 with two cylinders lopped off.
But both Citroen and Maserati will tell you that the engine in question was a unique design just for the SM. The SM would debut in 1970 with the 170 horsepower 2.7-liter V6, which would later also find a home in the Maserati Merak, albeit with slightly more power. The car used an oleopneumatic suspension system, similar to that of the DS. It also used hydraulic variable assist power steering and the same terrifying on/off hydraulic brake system which Citroen has since, thankfully, abandoned. It had headlights that swiveled with the front wheel direction, like the DS, but the SM had a total of six headlights, with only the inner two actually swiveling.
Though the engine was built by Maserati, the transmissions offered were built by Citroen, and are reportedly vastly superior to those used by Maserati for the same engine. The origins of the SM name have been lost to history, but one theory is that it stood for "Sport Maserati", an apt but boring word-pairing for the car. Another, more colorful, theory is that it stands for "Sa Majeste", French for "her majesty", a play on the DS's goddess nickname. In keeping with the sexualized car theme, there is also the connection to S&M, but this is highly unlikely to have actually been a factor in the car's naming.
If that's your thing, you're welcome to believe it, we don't really know, after all, but you'll likely be in the minority. There is also a certain sexual quality to the bodywork. Much of the DS's curvaceous design cues have carried over, but the sleeker and sportier design of the SM has made these curves more exotic and less bloated. The sporting nature of the car also helps with this, even if this is less obvious just from looking at it. The blend of the performance engine and the supremely comfortable ride has been said to possess a quality of the blend of masculine and feminine. This blend is present in all of the most sexualized cars, and it was something Citroen used to do quite well.
The SM never sold in huge numbers, but it was never really meant to either. Still, Citroen was losing a lot of money during this period, and after being bought out by Peugeot in 1974, the new parent company got rid of Maserati and killed off the SM after 1975. Many of the design cues would carry over into the CX, a far more successful car, but also not nearly as fast or special. It is still missed, and for a variety of different reasons.