Green’s Dictionary of Slang

1. uninhibited, lively, fashionable; esp. in phrs. swinging London, swinging ’60s.

1938		P. Grey ‘’Twixt Night ’n’ Dawn’ in Afro-American (Baltimore, MD) 12 Nov. 11/5: The club wonderland is rapidly gaining the reputation of being Baltimore’s swinging-est club uptown.
a.1955		‘Good-Doing Wheeler’ in D. Wepman et al. Life (1976) 75: Since he’d been gone she’d come on strong / And was one of the swingingest broads on the set.
1959		C. MacInnes Absolute Beginners 57: [He] struck me as not being a swinging character like his brother.
1965		Weekend Tel. 16 Apr. 12/2: [US speaker] Diana Vreeland [...] editor of Vogue [...] has said simply ‘London is the most swinging city in the world at the moment’.
1968		J. Barlow Burden of Proof 18: Swinging London and all that scene?
1972		G.V. Higgins Friends of Eddie Coyle 12: They tell me there’s some hash floating around in the real swinging places.
1980		M. Sellers Leonardo and Others 56: Zuleika lived life to the full. She was a product of the swinging sixties. So what? So was I.
1988		D. Woodrell Muscle for the Wing 10: The swinging side of the St Bruno night world had been run as smoothly and nearly as openly as a pizza franchise.
1999		Observer Screen 4 July 6: Cage, a responsible single parent, saves swinging bachelor Smith’s life in the line of duty.

2. a general term of approval.

1949		L. Hughes Tambourines to Glory I vi: I had such a swinging little helper.
1957		Laurents & Sondheim West Side Story I i: When you’re a Jet, / You’re the swingin’est thing. / Little boy, you’re a man; / Little man, you’re a king!
1958		PADS XXX 47: Swingin’, the highest term of approval. May be applied to anything a jazzman likes, or any person.
1961		T. Taylor Baron’s Court All Change (2011) 15: As I was putting [on] a very swinging disc of the Bird’s on to aid my digestion,.
1962		(con. 1950s) H. Simmons Man Walking On Eggshells 189: ‘You dig that?’ ‘Yeh, man, that’s swinging.’.
1972		(con. 1950s) D. Goines Whoreson 139 That’s swinging, brother.
1978		R. De Christoforo Grease 118: It’s cool, Kenick. It’s swinging.

3. pertaining to drug use and culture.

1958

E. Gilbert Vice Trap 74: He was a swinging doctor, himself; you know he became a connection later?

4. aware, ‘on the ball’.

1963

in T.I. Rubin Sweet Daddy 37: I’m all clued in. In here [i.e. prison] I’m swinging already. [...] Swinging, swinging. Making it go – working it – getting it.