William Hartson called the word “surely one of the ugliest words ever to slither its way into our dictionaries”, but regardless of what he would like to say about the word, I actually have always found it quite delightful, and others must have thought so too since seeing as it became so widespread.
I have heard in sources that it derived from college slang, and Online Etymology Dictionary puts:
also humungous, by 1972, American English, apparently a fanciful mash-up of huge and monstrous.
I found the following use in 1969 in "Princeton Alumni Weekly, Volume 70."
The response was humongous ( Gargantuan in size . Bigger than big )
In the example since the author is seen to feel a need to define the word surely leads one to assume that the word was exceptionally infantile to the English tongue.
In a 1967 book titled Current Slang the word is defined:
Humongous , adj . Heavy , large
And it looks as though by 1970 the word was being used freely and does not infer that a reader might be unfamiliar with the word's meaning.
I cannot find any records on who coined the word, and if so, where did it first appear? It is such a "new" word, that I doubt there cannot be found the first coining of it.
Best Answer
J.E. Lighter, Random House Historical Dictionary of American Slang (1997) has this entry for humongous:
The earliest match I've been able to find for the word (in the variant spelling humungus) is from a fraternity party advertisement in the [University Park, Pennsylvania] Daily Collegian (February 25, 1966):
However, supersleuth EL&U participant JEL has uncovered an even earlier instance of the variant humongus in Janice Higginbotham, "Students Display Varied Expressions and Sentiments," in the [Milledgeville, Georgia] Colonnade (March 17, 1964):
The Colonnade was the student newspaper of Woman's College of Georgia, now known as Georgia College and State University. In 1965 it had an enrollment of 1,002 students. Two months later, as EL&U participant shoover points out in a comment beneath this answer, a parody issue of the same student newspaper includes an instance of humongous. From Gwinn Leverett, "Please, De-Bees," in the [Milledgeville, Georgia] Columnude (May 29, 1964):
Evidently, humongus/humongous (very likely pronounced with a short o rather than a short u in the second syllable) was in reasonably widespread use at this college in Georgia by the end of the 1963–1964 school year.
An instance of humungous appears in another student newspaper—this time the one at Hardin-Simmons University in northern Texas—in late 1967. From Gary Stratton, "Coogs Take Eagles to Flush Season," in the H-SU Brand (November 21, 1967):
We thus have five instances from 1964 to 1967—from a small college in Georgia, a large university in Pennsylvania, a small college in Iowa (cited in user121863's answer), and a smallish regional university in Texas. Geographically, that represents a considerable range of occurrence—and underscores the origin of the word in student use.
Humungus also appears in Glen Gainsbrugh & Peter Whitehead, Two Travel Through, Or, The Skinny Shall Inherit the Earth (1968) [combined snippets], where the narrator is evidently a student, but the publisher is a major imprint of the time, Signet Books: