Troilism is the act of two people performing intercourse while a third watches. A common example is a husband watching his wife have intercourse with another man. This scenario is part of the swinger lifestyle, and varies depending on the participants.
Two of the parties are related, such as a married couple. The third party comes from outside the relationship, and can even be a stranger to the couple. Although the couple is usually separated, it is possible for the third party to be the observer.
Like many parts of the swinger lifestyle, there are no set rules for performing troilism. Sometimes the observer pleasures himself, other times he waits for delayed gratification; sometimes he watches from a few feet away, other times he watches in another room through a video camera. Another variant of troilisms involve hiring a prostitute for the act or prostituting one member of the couple. Illegal variants of troilisms is if one member of the sex act not knowing a third party is watching, and rape or forced participation of any combination of the parties. These illegal acts can often be role-playing scenarios.
Troilism was first coined in the 1941 edition of Dorland’s Medical Dictionary where it was classified as a paraphilia. It was not clear why this scientific name was chosen.
One examination of the word may indicate a root in the French word trois (three). A similar French term, triolisme, exists in french - the shift between the third and fourth letter is perhaps a mistake. On the other hand, ménage à trois (household of three) was coined in the late 19th century. Although all those terms involve three people, ménage à trois implies a romantic link between all three, which is quite different from troilistic scenarios. In French, a "plan à trois", vulgar form and synonym of "triolisme", corresponds more closely to a troilistic scenario; a "threesome".
Another possible origin was noted in Take Our Word For It, a webzine published by the non-profit Institute for Etymological Research and Education. Here, it is theorized that troilism comes from Shakespeare's Troilus and Cressida. In it, Ulysses forces Troilus to watch his lover, Cressida, with another man. Troilus promptly dismisses his former love as a whore. (Prostitution or “acting like a whore” is a common aspect of many acts of troilism.)