Green’s Dictionary of Slang

1. (US) used to indicate excitement, appreciation.

1944		E. Wilson [synd. col.] They whistled and gave the wolf’s woo-ooo-ooo cry when the chorus girls clacked through.
1945		E. Wilson I Am Gazing Into My 8-Ball 34: A guy can hardly go ‘Woo, woo!’ over somebody’s bosom before a circulator of this anatomical gossip sneaks up behind you and cries, ‘Hey, for God’s sake, don’t you know she wears Gay Deceivers?’.
1951		E. O’Connor Oracle 22: ‘That, my dear, was Christopher Usher [...] The naughty man is friendly with Miss Andriescu in 1405.’ ‘Woo woo,’ said the bellhop, rolling wide adolescent eyes.
1981		J. Ellroy Brown’s Requiem 23: Fat Dog [a golf caddy] [...] pulled out his roll. [...] There must have been sixty or seventy C-notes. [...] ‘Woo! Woo!’ I said. ‘I’m ditching this racket and becoming a caddy!’.
1994		W.D. Myers Darnell Rock Reporting 105: ‘Do you want boys to come into the girls’ locker room?’ [...] ‘I don’t care!’ she said. Then a lot of the boys started making noises like ‘wooo-wooo!’ and a few whistles.
2001		W.D. Myers Bad Boy 53: Lorelle Henry, was leading the group, and she was pretty, so we sent a few woo-woos her way.
2018		W. Kramer Hard Stuff 52: [R]ealizing that [girls] wanted to have sex, too [...] was a revelation. Woo-woo!

2. (US) used as a derisory excl.

1998		K. Anderson Night Dogs 338: ‘Woo woo,’ Hanson said, fanning the air with his hand, ‘Getting his orders from the radio’.
2023		(con. 1962) J. Ellroy Enchanters 141: Woo-woo. Motel Mike toasts four beaners [...] and it goes to his head.

3. (US) pertaining to unconventional, often ‘new age’ beliefs (e.g. wellness, crystal magic, aura-chakra cleaning, etc) that promote a succession of short-lived fads but have no scientific legitimacy.

2010		R. Skloot Henrietta Lacks 7: I was a science journalist who referred to all things supernatural as ‘woo-woo stuff’.
2018		Oprah Mag. 2 Oct. 🌐 Do we really need the ‘woo-woo’ fads we keep seeing on social media in our self-care routines? We put buzzy treatments from halotherapy to facial yoga to the test.