I thought that was the noise Curly Howard makes.
Heh. Oddly enough, it’s also used as lighthearted shorthand in some Pagan communities. For example, “Yeah, I was applying everywhere and not getting a single nibble, then I did some pro-employment woo, and got 3 calls back in one day!”
In that context, it’s “woo” or more often “woojie.”
And of course, there is always the Wapani-Wu.
Apparently a Woo Woo is an alcoholic beverage. Considering what some woo folk believe, I think this is the most likely origin of the term.
I had a friend who as a child was taught by her mother that the name for her girly bits was her “woo.”
Oh let’s not go there again…
I’m really old; I’ve been there for the last time.
Here’s the [thread=567685]'Is this hate speech?/ (woowoo means vagina)[/thread] thread, for those of you who were lucky enough to miss it the first time.
I’ve yet to hear woo in conversation. Feels like a strange word to actually speak, a bit like woot.
I’m used to the definition given by Acid Lamp, but “How I Met Your Mother” has warped how it’s pronounced in my head into a sort of Valley Girl-ish “WEEEUUUUUU!!!”
…I guess that whole “chugga-chugga…chugga-chugga” meme is just right out then? 
“…more coal! We’re coming up on some big hills…! Shovel like your life depends it!!!”
Speaking the word “woo” is often accompanied by air quotes.
I thought it derived partially from early Daffy Duck. http://www.youtube.com/watch?v=CukvlOL_2WA&t=5m21s
Admittedly, though, he does say “woo-hoo”.
This link had my understanding – New Age stuff – pyramid power, harmonic convergence… Also TM. EST, primal scream, and a bunch of other “therapeutic” practices.
Steely Dan asked of their song-character Dr. Wu:
“Are you with me Doctor Wu
Are you really just a shadow
Of the man that I once knew
Are you crazy are you high
Or just an ordinary guy
Have you done all you can do
Are you with me Doctor”
But in the final stanza answered in the negative:
“Are you with me Doctor Wu
Are you really just a shadow
Of the man that I once knew
She is lovely yes she’s sly
And you’re an ordinary guy…”
So it would seem that Dr. Wu isn’t all that crazy after all. In any event, his eccentricities would probably be outstripped by those of the eponymous “Josie”…
I usually go with a dismissive hand wave.
Periodic table of woo.
Wow, that’s about the most perfect chart ever!
And he wins his own thread! WOO HOO! 
Now, don’t go confusing your woo woo with your hoo hoo!