If you call someone masochistic, you are implying that they get pleasure from pain. Masochistic might be a good adjective to describe your friends who view participating in an Ironman Triathlon as their idea of a good time.
In the early 19th century, an individual with the last name Sacher-Masoch had some deviant, or abnormal, thoughts regarding sexuality. His name became the German noun Masochismus and, eventually, the English adjective masochistic. Sometimes masochistic is used incorrectly or as slang to refer to someone who just does something mean, as in "taking the wings off of flies is masochistic," implying that the person enjoys the pain of another, when the word really is specific to the person receiving pain. (It's a sadistic person who enjoys inflicting pain on someone else.)