Green’s Dictionary of Slang

swinging adj.3

[swing v. (8)]

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

[US]S. Harris Puritan Jungle 144: A new swinging couple — novices to the swinging rites.
[US]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.
[US]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.

[UK]Otway Atheist III iii: [ref. is to whoring] We are like to have a swinging time on’t.
[US]J. Rechy Numbers (1968) 109: [of a gay bar] Pershing Square and Main Street and its two swinging bars.
[US] 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?

In compounds

swinging single (n.) (also swingle)

(US) a sexually promiscuous unmarried person.

[US]D. Pendleton Executioner (1973) 14: Residence club, it’s called – for swinging singles only.
[US]J. Thompson ‘Sunrise at Midnight’ in Fireworks (1988) 161: Domiciles [...] devoted exclusively to the young – the ‘swinging singles,’ so called.
[US]Newsweek 16 July 53: The sheer number of singles, meshed with the media’s seductive imagery (singles who swing are jauntily dubbed ‘swingles’), is gradually revising society’s view of its unwed members.
[UK]J. McClure Spike Island (1981) 200: He has a ham roll in a club for swinging singles.
[US]M.B. Sussman et al. Handbk Marriage 312: Although the ‘swinging singles’ stereotype provides good movie material, few singles have the inclination or motivation to ‘swing’ for any length of time.