The word swinging was previously called wife swapping, people engaging in this act were also called wife swappers and gained attention from the media in the 1950s.
Once the media attention grew the wife swappers sought for a better term to describe their activities. The Activity of wife swapping was changed to swinging, and in recent years the Swinglifestyle.
Swinging or multiple partners started thousands of years ago during Roman Orgies. It was not uncommon for wives and husbands to invite friends to an orgy to swap out partners. In Ancient Egypt during similar events, they would drink wine soaked with Blue lotus as an aphrodisiac to heighten sexual alertness and sensuality.
Swinging in the U.S supposedly started in the Air Force during World War II. The pilots were rich enough to move their wives closer to the base. Since the fatality rate of a pilot was high the social circle changed into being accepted in which non monogymy of other pilots were highly accepted. Look at the movie Pearl Harbor, when one pilot is assumed dead, the wife moves on with his friend. These accepted swinging arrangements continued throughout bases during World War II and even up into the Korean War as well.