It seems based on an interjection common in 19th century accounts of American Indians (though I did find one about New Guineans) a sort of whooping. Here's an interesting example from Moby Dick (ch 61):

 Quote:
"Woo-hoo! Wa-hee!" screamed the Gay-Header in reply, raising some old war-whoop to the skies; as every oarsman in the strained boat involuntarily bounced forward with the one tremendous leading stroke which the eager Indian gave.

The Gay-Header referred to is Tashtego a Native American harpooner (ch 27):

 Quote:
Next was Tashtego, an unmixed Indian from Gay Head, the most westerly promontory of Martha's Vineyard, where there still exists the last remnant of a village of red men, which has long supplied the neighboring island of Nantucket with many of her most daring harpooneers. In the fishery, they usually go by the generic name of Gay-Headers.

Yahoo (or yah hoo) can also be an interjection of joy or exultation, besides being the name of a people in Gulliver's Travels.

Ceci n'est pas un seing.