Years before the Internet company Yahoo! came into existence, the Indian guru Bhagwan Shree Rajneesh was urging use of the word as a more hygienic form of greeting: "When you come across a friend, raise your hands to the sky and scream 'Yaa Hoo' instead of employing the universal handshake."
The Molina Dispatch – Apr 1, 1988
Apparently Rajneesh believed that "Yaa-Hoo" was quite a powerful word, since he also had his followers use it in a ritualized laughter therapy:
The first part will be Yaa-Hoo!—for three hours, people simply laugh for no reason at all. And whenever their laughter starts dying they again say, "Yaa-Hoo!" and it will come back. Digging for three hours you will be surprised how many layers of dust have gathered upon your being. It will cut them like a sword, in one blow. For seven days continuously, three hours every day… you cannot conceive how much transformation can come to your being.
And then the second part is "Yaa-boo." The first part removes everything that hinders your laughter—all the inhibitions of past humanity, all the repressions. It cuts them away. It brings a new space within you, but still you have to go a few steps more to reach the temple of your being, because you have suppressed so much sadness, so much despair, so much anxiety, so many tears—they are all there, covering you can destroying your beauty, your grace, your joy.