“Shit hits the fan” (SHTF) means that something is going to cause a great stink (or great trouble). The expression was popularly used by American soldiers in World War II, especially during the Guadalcanal Campaign (1942-1943). ““The garbage hit the fan on that one” was printed in a 1945 book and “the mud hits the fan” was printed in a 1945 magazine. ““Jesus, boys, come on, the shit’s hitting the fan” was printed in a 1947 novel.
 
“SHTF” has been cited in print since at least 1998. Many people and businesses have disaster or “SHTF” preparedness plans.
 
   
Wiktionary: hit the fan
Etymology
This expression is almost certainly from colloquial expressions involving shit hitting the fan.
Verb
hit the fan (third-person singular simple present hits the fan, present participle hitting the fan, simple past and past participle hit the fan)
1. (idiomatic, intransitive) To have a dramatic, usually negative, effect.  
Usage notes
Use with other subjects than shit is probably intended to evoke the image without actually saying the taboo (in some contexts) word.
     
Google Books
The U.S. Marines on Iwo Jima
By Raymond Henri, Jim Griffing Lucas, W. Keyes Beech, David Dempsey and Alvin M. Josephy
New York, NY: Dial Press
1945
Pg. 25:
“The garbage hit the fan on that one,” said a captain.

Google Books 
Skyways for Business
Volume 4
1945
Pg. 59:
The ones you sweat out turn out to be soft touches, and the ones that look like gravy hops are where the mud hits the fan.
 
Google Books
The Conqueror Comes to Tea;
Japan Under MacArthur
By John La Cerda
New Brunswick, NJ: Rutgers Univ. Press
1946
Pg. 104:
Written in Parrott’s room— which came to be known as “the snake pit”— this was the song:
THE SHINTO HIT THE FAN.
 
Google Books
Class Report:
A Novel
By John Francis Purcell
New York, NY: Vanguard Press
1947
Pg. 52:
“Jesus, boys, come on, the shit’s hitting the fan.”
   
Google Books
The Naked and the Dead
By Norman Mailer
New York, NY: Rinehart
1948
Pg. 573:
“When the shit hits the fan that’s when you keep a…”
   
Google Books
The Old Breed;
A History of the First Marine Division in World War II
By George McMillan
Washington, DC: Infantry Journal Press
1949
Pg. 145:
... Guadalcanal campaign was the epitome of the common Marine expression, “the stuff hit the fan,” an expression so popular on Guadalcanal that it was used as code in telephonic conversation to mean that there was action, a fight.
 
6 October 1950, Boston (MA) Traveler, Robert C. Ruark column, pg. 47, col. 6:
He is, secondly, the kind of long-cut sneerer at his land who deserts it while exploiting it, and then rushes back to mama’s apron when the stuff hits the fan.
 
Google Books
Moth in a Rag Shop
By Robert Chambers
Indianapolis, IN: Bobbs-Merrill Company
1968
Pg. 98:
“When that big bucket of shit hits the fan, man, and the rifles are popping from the roofs like it was the Fourth of July, and Avenue C starts to be a real fine place to roast a couple million weenies, and the National Guard goons move in with tanks ...”
 
OCLC WorldCat record
Editorial: The “apathy until it hits the fan syndrome”.
Edition/Format: Article Article : English
Publication: Journal - California Dental Association, 1973 Nov; 1(5): 8-9
Database: From MEDLINE®/PubMed®, a database of the U.S. National Library of Medicine.
 
OCLC WorldCat record
When the shit hits the fan
Author: Master Boogies Song and Dance.; Cornell University Library Hip-Hop Collection.
Publisher: New York, N.Y. : West End Records, 1980.
Edition/Format:   Music LP : English
 
OCLC WorldCat record
When it hits the fan : managing the nine crises of business
Author: Gerald C Meyers; John Holusha
Publisher: New York : New American Library, 1987, ©1986.
 
Google Groups: alt.music.replacements
Shit Hits the Fans Plus
Anthony LaRosa
5/18/98
(...)
It’s “SHTF” plus “Westerberg Demos” (i.e. “Man Without Ties” and “Seein’ Her”) to fill up the CD.
 
Google Groups: rec.guns
What is the best semi-auto rifle when the SHTF?
Arthur Luxury-Yacht
7/4/01
I know this was debated pre-Y2K but was wondering what would be a good choice. What is the best choice rifle for for TEOMAWKI or when the sh*t hits the fan
   
OCLC WorldCat record
The prepper’s handbook : a guide to surviving on your own
Author: Zion Prepper
Publisher: [Charleston, SC] : [CreateSpace], ©2011.
Edition/Format:   Book : English
Contents:
What Do We Do If There Is a SHTF Situation?—