The noun yahoo denotes a rude, noisy or violent person—cf. also bully.

It was invented by the Irish satirist, poet and Anglican cleric Jonathan Swift (1667-1745) as the name of a race of brutish creatures resembling men, ruled by the houyhnhnms, described as horses endowed with reason, in Travels into several remote Nations of the World. In four Parts. By Lemuel Gulliver, first a Surgeon, and then a Captain of several Ships (London, 1726):

(1760 edition)
The fore-feet of the yahoo differed from my hands in nothing else, but the length of the nails, the coarseness and brownness of the palms, and the hairiness on the backs. […]
The sorrel nag […] brought out of the yahoo’s kennel a piece of ass’s flesh, but it smelt so offensively that I turned from it with loathing; he then threw it to the yahoo, by whom it was greedily devoured. […] As to those filthy yahoos, although there were few greater lovers of mankind, at that time, than myself; yet I confess, I never saw any sensitive being so detestable on all accounts; and the more I came near them, the more hateful they grew, while I stayed in that country.

The servants drive a herd of Yahoos into the field laden with hay
by Louis Rhead (1857-1926)
image: The Metropolitan Museum of Art, New York

 

The earliest transferred, allusive use of yahoo that I have found is from Dialogues of the Dead, published in The Public Advertiser (London) of Monday 26^th May 1760:

When any Hero comes hither from Earth who wants to be humbled (as most Heroes do) they [= Pluto and Proserpine] should set Swift upon him, to bring him down. The same good Office he may frequently do to a Saint swoln too much with the Wind of spiritual Pride; or to a Philosopher vain of his Wisdom and Virtue. He will soon shew the first, that he cannot be holy without being humble; and the last, that, with all his boasted Morality, he is but a better kind of Yahoo.

In An Inquiry into the Human Mind, on the principles of Common Sense (Edinburgh, 1764), the Scottish philosopher Thomas Reid (1710-96) wrote:

(1872 edition)
Zeno endeavoured to demonstrate the impossibility of motion; Hobbes, that there was no difference between right and wrong; and this author, that no credit is to be given to our senses, to our memory, or even to demonstration. Such philosophy is justly ridiculous, even to those who cannot detect the fallacy of it. It can have no other tendency, than to shew the acuteness of the sophist, at the expense of disgracing reason and human nature, and making mankind Yahoos.

In The spiritual Quixote: or, The summer’s ramble of Mr. Geoffry Wildgoose (London, 1773), by the English author and translator Richard Graves (1715-1804), some men are seen as yahoos, as opposed to animals:

“One feels the most for those animals that are tortured and abused. But, I think, none are more so, than the generality of horses and beasts of burthen, from a want of sensibility in the reasoning brutes to whose care they are usually intrusted. You saw how little compassion that Butcher’s lad has shewn to his loaded Steed. And to see a noble creature start and tremble at the passionate exclamation of a mere Yahoo of a stable-boy; who, if he knew his own strength, could drive a dozen men before him; I own, equally excites my pity and my indignation.”

The word came to also mean orang-utan, or wild man of the woods; for example, on Friday 22^nd April 1814, The Lincoln, Rutland, and Stamford Mercury (Stamford, Lincolnshire) published an advertisement for “T. Shore’s superb collection of living rarities”, which included:

The Great Yahoo, or Wild Man of the Woods.

The word yahoo, or yowie, denotes a large, hairy, man-like creature, probably mythical, supposedly inhabiting south-eastern Australia; in this instance, yahoo might have an entirely different origin — or might be a transferred use of yahoo in the sense of orang-utan, according to Graham Joyner in The orang-utan in England: an explanation for the use of Yahoo as a name for the Australian hairy man.

 

The exclamation of great joy or excitement yahoo is unrelated to the word coined by Jonathan Swift: it is a natural interjection. The following image is from Skippy, a comic strip by the American author, illustrator and cartoonist Percy Lee Crosby (1891-1964), published in the St. Louis Post-Dispatch (St. Louis, Missouri) on Sunday 16^th August 1936: