"Woo" is the recently invented slur of choice embraced by proponents of "New Atheism," the brand of unbelief which clings to reductionism, which makes no distinctions between the many different breadths and depths of religious and spiritual belief, and which fuels itself with stringent mockery of such belief. It is a term often assertedly addressed to ideas importing spirituality to contexts outside of organized religions, though it seems applicable to-- well, anything the atheist finds disagreeable. But don't take my word for it. Here's are some atheists' own descriptions of the meaning and use of the term:
What is Woo, you ask? Woo is is chakras, vibrations, and astrology. Woo is metaphysical silliness that uses words like ‘energy’ (but not even close to it’s physics meaning) or even worse ‘energies’. Woo is the New Age, or as some call it, ‘newage’ (rhyming with ‘sewage’).

Another writes:
Woo, woo-woo, Oga Boga, POOF!ism, and my favourite Oogity Boogity! (Blessed be His name). Many people are deeply offended to have their most cherished beliefs classified this way. .... Calling any kind of faith-based belief that people are willing to act on with the pejorative term ‘woo’ is a good place to start.

And another:
Woo beliefs are untested and untestable at best; tested and demonstrably false at worst. And basing your life on a false premise is going to lead to you bad decisions. Garbage in, garbage out, as the data processors say.

Now, if the atheist wishes to proclaim that belief in crystals or horoscopes or sending positive feelings through the void is unscientific, that's their business. But a pejorative, once established as pejorative, inevitably creeps beyond that. "Woo" becomes the crutch, the lazy way to dismiss ideas without having to give them fair consideration. Interestingly, a much older instance of this sort of dismissiveness is evident in the criticisms initially leveled against Albert Einstein's theory of relativity (Ironically, Max Bernhard Weinstein, one of the earliest proponents of Pandeism, was one of the most stringent critics of the theory of relativity, contending that it "had removed gravity from its earlier isolated position and made it into a "world power" controlling all laws of nature"); and likewise of Georges Lemaître's theory of an initial expansion of our Universe, critically labeled the Big Bang by detractor Fred Hoyle (though Hoyle insisted that he coined that phrase not as a pejorative but as a contrast against what he believed to be the more reasonable explanation his steady state theory).

And so it is with the appendment of "woo" against whatever is deemed outside the sphere of what is properly atheistic to believe. The experiences of meditational mysticism? Dismissed as "woo." Breathtakingly uncanny coincidences, instances of emotionally bonded by physically separated people having the same sensation at the same moment? Shrugged off with those same three letters. Assertions even by PhD astrophysicists suggesting that there may be more to our Universe than can be explained by the natural sciences? Waved away with a word. And all this in the context of claiming that the intricately elegant Standard Model of particle physics simply is, as a brute fact, and in observing the strange scientific discoveries that particles half a Universe apart can have a quantum entanglement such that one responds to effects on the other instantaneously, and where particles are found to act differently upon coming under the view of a conscious observer. Yes, the atheist is free to label as "woo" whatever he wishes, but when the labeling acts as a substitute for reasoned consideration of the possibilities, then the dispensation of that label is itself no more rational than the things being labelled by it.