What is the origin of this word? How did it come to be synonymous with skeptics?
"Woo" (or "WooWoo") seems to refer to the types of "magical thinking" that skeptics love to, well, be skeptical about.
From Respectful Insolence, this thought on its origins:
Finally, regarding the etymology, I tried to look into that a bit. I do know that The origin of "woo woo" is lost in the mists of time (well, at least decades) of time. I've seen suggestions that it comes from the "woooooo" noise that a Theramin used to make in old horror or science fiction movies. I've also seen suggestions that it somehow derives from the music in the theme to The Twilight Zone. Whatever the etymology, the term can, as far as I can tell, only be traced back to around 1986, at least in print, although I'd be shocked if the term wasn't in use long before that. However, I can't remember having heard the term until more recently, within the last few years. Maybe I was just sheltered.
Personally, +1 on the Theremin idea.
Woo-woo (or just plain woo) refers to ideas considered irrational or based on extremely flimsy evidence or that appeal to mysterious occult forces or powers.
Here's a dictionary definition of woo-woo:
adj. concerned with emotions, mysticism, or spiritualism; other than rational or scientific; mysterious; new agey. Also n., a person who has mystical or new age beliefs.
When used by skeptics, woo-woo is a derogatory and dismissive term used to refer to beliefs one considers nonsense or to a person who holds such beliefs.
Sometimes woo-woo is used by skeptics as a synonym for pseudoscience, true-believer, or quackery. But mostly the term is used for its emotive content and is an emotive synonym for such terms as nonsense, irrational, nutter, nut, or crazy.
See also discussion on The Lippard Blog re: a 1984 citation:
THE NEW AGE SOUND: SOOTHING MUSIC BY SINCERE ARTISTS Philadelphia Inquirer, The (PA) - Sunday, October 21, 1984 So who is this New Age audience? Mostly upscale folks in their 30s and early 40s, the ones weaned on Baba Ram Dass and Woodstock and hallucinogenics, macrobiotic diets and transcendental meditation. ..... George Winston, who practices yoga and who currently has three albums on the jazz charts (his five Windham Hill recordings have reportedly sold more than 800,000 copies; his LP December has just been certified gold), has jokingly called this crowd the "woo-woos." In a 1983 interview in New Age Journal, Winston, asked if he knew who comprised his audience, answered that there were some classical fans, some jazz, some pop and "all the woo-woos." "You know," he added, "there's real New Age stuff that has substance, and then there's the woo-woo . A friend of mine once said, 'George, you really love these woo-woos, don't you?' and I said 'Yes, I do love them,' and I do. I mean, I'm half woo-woo myself."
First, it’s a term coined relatively recently, and it is only used in certain circles. It is not present in generic well-known dictionaries (Merriam-Webster, Collins, Oxford or Cambridge). Wiktionary defines it, but relies on rather nonmainstream sources.
Secondly, woo is the shortened form, and it is also written as woo-woo and woo woo. Wiktionary says of its origin that it might be “intended to imitate the eerie background music of sci-fi/horror films and television shows, however the exact origin is uncertain”. Indeed, if you do a Google Books search for “woo woo”, all the hits refer to the sound of the wind at night, animal calls (owl, wolves, etc.).
I posed this exact question to DARE and the American English Twitter groups. Got a citation from the '60s in return, so it goes way back. Often expressed as "woo-woo." Wide geographic distribution from west to east coasts.
In horror movies, whenever the monster or something scary is due to appear, the event is often preceded (at least in older films) with eerie, "woo-woo-woo..." music. That might be a possible origin but at this time it's one of those etymological unknowns.
Terence McKenna was a radical thinker. He was a classically trained biologist and wrote what he referred to as 'straight' ethnobiology pieces and also what he referred to as being more 'woo woo'. The types of things I'm thinking of are epitomized by Timewave Zero.
Basically, the way he used it, is that something would be 'woo woo' if it was an idea that wouldn't pass muster on a site like this.