Green’s Dictionary of Slang

1. referring to those who participate in husband-and-wife swapping parties or those who enjoy a promiscuous sex life.

1969		S. Harris Puritan Jungle 144: A new swinging couple — novices to the swinging rites.
1974		small ad in J.R. & L. Smith Beyond Monogamy 247: Couple, attractive, 26 and 25, who have not yet swung, desire to meet swinging couple.
2007		M. Coleman Family Life in 20th-Century America 277: An ad placed by a swinging couple might read: Lexington, Kentucky marrieds. Attractive, college, white, want to hear from other attractive marrieds.

2. sexually active; used for sexual contacts.

1683		Otway Atheist III iii: [ref. is to whoring] We are like to have a swinging time on’t.
1967		J. Rechy Numbers (1968) 109: [of a gay bar] Pershing Square and Main Street and its two swinging bars.
1967		in S. Harris Hellhole 211: [of a lesbian bar] Lillian, is that the name they call you by in that swinging place you hang in, man?
1974		E. Wilson Show Business Laid Bare 57: John F. Kennedy [...] was a giant in sex. [...] I truly believe he would have approved my contention that he was the sexiest, swingingest President of the century.