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.