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Yahoo
During Swift’s “A Voyage to the Country of the Houyhnhnms”, the term “yahoo” is frequently used to describe “an odious Animal, for which (Gulliver) had so utter an Hatred and Contempt” (201). The Houyhnhnms are superior to the Yahoos in every way though they are horses, and Yahoos bear the likeness of humans, save some physical features. The text leads the reader to conclude that the Yahoos are abominable and filthy.
OED:Yahoo n.- 1. A name invented by Swift in Gulliver's Travels for an imaginary race of brutes having the form of men; hence transf. and allusively, a human being of a degraded or bestial type. (Cf. houyhnhnm n.) Freq. in mod. use, a person lacking cultivation or sensibility, a philistine; a lout, a hooligan.It was so interesting to see the word evolve in its use and increase in its exponentially negative connotation:
1726 Swift Gulliver II. iv. ii. 23 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. 1764 T. Reid
Inq. Human Mind i. §5. 28 At the expence of disgracing reason and human nature, and making mankind yahoos. 1861 H. Kingsley
Ravenshoe lv, ‘And what sort of fellow is he?..a Yahoo, I suppose?’ ‘Not at all; he is..a perfect gentleman.’ 1904 C. R. Ashbee
Last Rec. Cotswold Community p. xxiii, To have..the pleasant valleys of Saintbury and Weston tramped by armed bands of Birmingham yahoos. 1912 J. Sandilands
Western Canad. Dict., Yahoo, a lout from the back-country, an ignoramus, a know-nothing. 1968
Courier-Mail (Brisbane) 21 May 6/4 A Brisbane boat owner has complained that ‘young yahoos’ are stripping cars left at bayside boat ramps. 1977
New Yorker 20 June 56/2 Sarge Waller—who is, among other things, a professional riverman—later commented on Cook and Ulvi's journey and described them as ‘yahoos’. attrib. Source: http://libproxy.library.unt.edu:2200/view/Entry/231128?rskey=rTTYpv&result=1#eid ECCO: Yahoo appeared in a couple of texts between 1720 and 1730 besides Swift’s writings. It appeared under Religion and Philosophy: TITLE: A brief concordance or table to the Bible of the last translation. Serving for the more easy finding out of the most useful Places therein ... AUTHOR: Downame, John.
DETAIL: London, MDCCXXVI. [1726]. 77pp. Religion and Philosophy
And of course, Literature and Language:
TITLE: #A proper reply to a lady, occasioned by her Verses address'd to the imitator of the first satire of the second book of Horace. By a gentleman. AUTHOR: Gentleman.
DETAIL: London, [1733]. 7pp. Literature and Language

The Yahoos are considered as idiots throughout the text and always have been rendered under this interpretation since its numerous uses in "Gulliver's Travels". But, in terms of its use from the OED, it has become a term to describe unruly lawbreakers rather than uneducated, ignorant and ill-mannered people.