sadism: "love of cruelty," 1888, from Fr. sadisme, from Count Donatien A.F. de Sade (1740-1815). Not a marquis, though usually now called one, he was notorious for the cruel sexual practices he described in his novels. Sadist first recorded 1897; sadistic is 1892, after Ger. sadistisch. Compound Sado-masochism first recorded 1935 (see masochist). Abbreviation S & M first attested 1965. http://www.etymonline.com/index.php?term=sadism Late 19th century. < French sadisme, after the Marquis de Sade http://encarta.msn.com/encnet/features/dictionary/DictionaryResults.aspx?refid=1861726782 The development of the term sadomasochism is complex. Originally "Sadism" and "Masochism" were purely technical terms for psychological features, which were classified as psychological illness. The terms are derived from the authors Marquis de Sade and Leopold von Sacher-Masoch. In 1843 the Hungarian physician Heinrich Kaan published Psychopathia sexualis ("Psychopathy of Sex"), a writing in which he converts the sin conceptions of Christianity into medical diagnoses. With his work the originally theological terms "perversion", "aberration" and "deviation" became part of the scientific terminology for the first time. The German psychiatrist Richard von Krafft-Ebing introduced the terms "Sadism" and "Masochism" into the medical terminology in his work Neue Forschungen auf dem Gebiet der Psychopathia sexualis ("New research in the area of Psychopathy of Sex") in 1890. In 1905 Sigmund Freud described "Sadism" and "Masochism" in his Drei Abhandlungen zur Sexualtheorie ("Three papers on Sexual Theory") as diseases developing from an incorrect development of the child psyche and laid the groundwork for the scientific perspective on the subject in the following decades. This led to the first time use of the compound term Sado-Masochism (German "Sado-Masochismus")) by the Viennese Psychoanalyst Isidor Isaak Sadger in his work Über den sado-masochistischen Komplex ("Regarding the sadomasochistic complex") in 1913. http://en.wikipedia.org/wiki/Sadism_and_masochism Sadism often involves binding, spanking, whipping, and, in extreme cases, mutilation and murder. Among the many explanations of the causes of sadism is the theory that the sadist, most often a male with a low sense of self-worth since childhood, is trying to prove importance and superiority by punishing others. The term sadism, which is derived from the name of the Marquis de Sade, is also applied in a nonsexual, general sense to anyone who delights in cruelty. http://encarta.msn.com/encyclopedia_761563947/Sadism.html After Donatien Alphonse François de Comte Sade. http://www.thefreedictionary.com/sadism Donatien Alphonse-François de Sade (June 2, 1740 – December 2, 1814) (pronounced IPA: [maʁki: dÉ™sad]) was a French aristocrat and writer of philosophy-laden and often violent pornography. He was a philosopher of extreme freedom (or at least licentiousness), unrestrained by morality, religion or law, with the pursuit of personal pleasure being the highest principle. Sade was incarcerated in various prisons and in an insane asylum for about 32 years of his life—eleven years in Paris (10 of which were spent in the Bastille) a month in Conciergerie, 2 years in a fortress, a year in Madelonnettes, 3 years in Bicêtre, a year in Sainte-Pélagie, 13 years in the Charenton insane asylum—indeed, much of his writing was done during his imprisonment. The term "sadism" is derived from his name. http://en.wikipedia.org/wiki/Marquis_de_Sade After Comte Donatien Alphonse François, Marquis de Sade, French novelist, 1740–1814. http://www.mercksource.com/pp/us/cns/cns_hl_dorlands.jspzQzpgzEzzSzppdocszSzuszSzcommonzSzdorlandszSzdorlandzSzdmd_s_01zPzhtm#12717569

To paraphrase the other (much longer) answer it's based on the name of the late 18th - early 19th century French nobleman the Marquis de Sade.