Green’s Dictionary of Slang

swinging adj.3

[swing v. (8)]
[1960s+]

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

2. sexually active; used for sexual contacts.

In compounds

swinging single (n.) (also swingle)

[1960s+] (US) a sexually promiscuous unmarried person.