Of American-English origin, the adjective humongous, also humungous, means: extremely large, huge, enormous.

This word is of uncertain derivation; it is probably a factitious adjective coined on the suffix -ous, influenced by hugeous and monstrous, and perhaps by the stress-patterns of stupendous, tremendous, etc.

The adjective humongous occurs, for example, in the following tweet (i.e., message on the Twitter website), by Sonia Sodha, of The Observer (London, England), dating from Thursday 29^th November 2022—as reproduced in ‘Brutal’: media deride Liz Truss’s local radio interviews: Media commentators across political spectrum deride PM’s appearance on local radio to defend economic policy, by the political correspondent Andrew Sparrow, published in The Guardian (London and Manchester, England) of Thursday 29^th November 2022:

My god this is gaslighting on a humongous scale.

The earliest occurrences of the adjective humongous, humungous, that I have found are as follows, in chronological order:

1-: From the following advertisement, published in The Tampa Times (Tampa, Florida) of Thursday 20^th July 1967—Sep. DR is the abbreviation of separate dining-room:

IN LOVELY GOLFVIEW

ONE-Story & really a gracious home! The “gathering” room overlooks the nicest Garden. Of course there’s a Sep. DR, and all the other rooms are just “Humongous.”
MEL CODY Res. 255-8961

2-: From the Tallahassee Democrat (Tallahassee, Florida) of Wednesday 11^th September 1968:

—Florida Highway Patrol Photo

Arthur Vickers (l) and Jon Mortie Met Big Rattler
…4½-foot snake was killed with carbine; had 13 rattles

R.I.P., ‘Humongous’ Rattler

The last thing State Trooper B. R. Burkett expected when he walked up to a suspected stolen car on Lake Iammonia Road Tuesday afternoon was a man stepping out the underbrush asking, “Hey, you gotta gun?”
Muscles tensed and hand on his sidearm, Burkett nodded.
“Well, c’mon then,” the man said. “A ‘humongous’ rattler jest crawled between my legs.”
The man disappeared into the underbrush and Burkett grabbed the service carbine from his patrol car.
A few moments later, Burkett’s report said, “snake was dispatched with several shots from trooper’s carbine.”
The snake was hung across a fence as a warning to unsuspecting walkers. The car was classified as abandoned.

3-: From The Knoxville News-Sentinel (Knoxville, Tennessee) of Tuesday 19^th November 1968:

Spelunkers
SS Class Explores Big Cave
By JIM DYKES
The Bearden Central Baptist Youth 5 class, most about 15 years old, with two adults and two knowledgeable young spelunkers, spent a weekend recently in Saltpeter Cave in Grassy Cove.
Class teacher Joe Crichton said the group of 13 went into the cave Saturday and explored until midnight. They dropped off camping gear in a vaulted room about a half-mile from the adit and managed to work their way back another mile and a half to a chasm in the passage.
It was six feet wide and about 70 feet deep. One of the young spelunkers, Ted Bailey, Kingston, “did the split” and went down the chasm until he could go no further. He didn’t use a rope. Ted had been in the cave three or four times previously. The other Kingston youth, Steve Street, had been in before also. Both the boys are active in scouting and experienced speleogists [sic].
The group decided not to chance a crossing of the fissure. After exploring other avenues and rooms, they returned to the campsite where, Crichton says, “half went to sleep and the other half played hide-and-seek half the night through the passageways.”
The other half woke to find “humongous” (that’s Steve Street’s word; same one he uses to describe the cave) cave rats trying to get their food. They rigged rock deadfalls to be tripped with strings but went back to sleep shortly before the rats returned and stole the bait.

4-: From the following advertisement, published in The Montgomery Advertiser (Montgomery, Alabama) of Thursday 6^th March 1969:

OLD GAY MEADOWS—The “Foothills of the McGehee Estates” where the homes lie on huge lots and sit elegantly 100 ft. or more from the street. Financial arrangements you would not believe!—But, you can afford. Den, eat-in kitchen, humongous living-dining combination, carport. Payments just like you want them. Down payment to suit YOU. Call Joe Harwell at 272-1105, or GIVENS & MCDOWELL, 272-7090.

5-: From Spring Fever Sweeps Over Southern, by Deb Bowen, a student at Southern High School, Durham, North Carolina, published in the Durham Morning Herald (Durham, North Carolina) of Saturday 19^th April 1969:

That potent formula for Spring Fever seeps into our veins and carries our minds far away from those four walled, multi-desked, one door, classrooms with their broken clocks, dirty blackboards, and bulletin boards that continue to display the remainders of football season or a Valentine smash.
Eyes wander through the open windows to those humongous (imaginary word) puffs of cotton that pretend to be castles or sheep . . . while they drift or dance through that endless maze of blue. Instructors make futile attempts to gain attention but dreamers go on dreaming.