The newspaper columnist and American language scholar H. L. Mencken (1880-1956) wrote in 1927 about the words he created:
 
“Of my inventions I am vainest of Bible Belt, booboisie, smut hound and Boobus americanus.”
 
The “Boobus Americanus” is the American boob—the average gullible American. The term has been cited in print since at least 1920.
 
   
Wikipedia: H. L. Mencken
Henry Louis “H. L.” Mencken (September 12, 1880 – January 29, 1956) was an American journalist, essayist, magazine editor, satirist, critic of American life and culture, and scholar of American English. Known as the “Sage of Baltimore”, he is regarded as one of the most influential American writers and prose stylists of the first half of the twentieth century. Many of his books remain in print.
 
Chronicling America
7 January 1920, New York (NY) Tribune, “Books” by Heywood Broun, pg. 14, col. 7:
“In the manner of Mr. Wells, one imagines Mr. Mencken the last to succumb. One fancies him dragged to the Altar of Knowledge still hissing, ‘Pishposh, Yokel and Boobus Americanus’ under his breath and still convinced that W. Allen White is the idol of Western America.”
 
Google Books
The American Credo:
A Contribution Toward the Interpretation of the National Mind
By George Jean Nathan and Henry Louis Mencken
New York, NY: Alfred A. Knopf
1921
Pg. 100:
Thus the Boobus americanus is lead and watched over by zealous men, all of them highly skilled at training him in the way that he should think and act.
 
Google Books
Prejudices:
Third Series, Volume 3
By Henry Louis Mencken
New York, NY: Alfred A. Knopf
1922
Pg. 16 (“On Being an American”):
The boobus Americanus  is a bird that knows no closed season—and if he won’t come down to Texas oil stock, or one-night cancer cures, or building lots in Swampshurst, he will always come down to Inspiration and Optimism, whether political, theological, pedagogical, literary, or economic.
 
Google Books
The Gist of Mencken:
Quotations from America’s Critic
Edited by Mayo DuBasky
Metuchen, NJ: Scarecrow Press
1990
Pg. 230:
NEOLOGISMS
Of my inventions I am vainest of Bible Belt, booboisie, smut hound and Boobus americanus.
[Letter (NYPL); to Charles Green Shaw, N.Y.; December, 1927.]
   
Urban Dictionary 
Boobus Americanus
Complacent, lazy & stupid US citizen who votes for politicians based upon information proffered by government shills in the mainstream media, or for other vacuous reasons. This dolt not only doesn’t understand the concept of “limited government”, but has never read The Constitution nor The Bill of Rights. He also believes his vote counts and that politicians are honest.
(...)
by GovernmentSucks Jun 28, 2009
 
Twitter
Desiree Morales‏
@verbadverb  
“Of my inventions I am vainest of Bible Belt, booboisie, smuthound and Boobus americanus. ”—HL Mencken http://bit.ly/l4FSer
2:55 PM - 7 Jun 11
 
Google Books
The Untold History of the United States
By Oliver Stone and Peter Kuznick
New York, NY: Gallery Books (Simon & Schuster)
2012
Pg. 43:
Mencken referred to the common man, mired in religion and other superstitions, as a “boob,” a member of the species “boobus Americanus.”
 
Twitter
Clark‏
@rtclark  
Boobus Americanus “@DRUDGE_REPORT: ‘DUCK DYNASTY’ Premiere Shatters Cable Records With 11.8 Million Viewers… http://drudge.tw/1dbkkVP ”
2:49 PM - 15 Aug 13