Where did this come from? It makes no sense to me...why is the shit even near the fan?
Partridge says it's US and Canada slang from c. 1930, and that Norman Franklin says (1976) the original reference is to ther agricultural muck-spreader, and also mentions the following joke as perhaps valid.
The Online Etymology Dictionary says:
The expression [the shit hits the fan] is related to, and may well derive from, an old joke. A man in a crowded bar needed to defecate but couldn't find a bathroom, so he went upstairs and used a hole in the floor. Returning, he found everyone had gone except the bartender, who was cowering behind the bar. When the man asked what had happened, the bartender replied, 'Where were you when the shit hit the fan?' [Hugh Rawson, "Wicked Words," 1989]
US military in WWII
The phrase was at least part of US military slang during World War II, as euphemistic versions can be found in contemporary books, particularly in US Marines accounts of the war. From 1945's The U. S. Marines on Iwo Jima by Raymond Henri et al.
"The garbage hit the fan on that one," said a captain.
The 1947 Star-Spangled Mikado by Frank Raymond Kelley says:
In December, 1945, to borrow a line from an irreverent song popular among Americans in Tokyo "the Shinto hit the fan."
The song title is also shown in the 1946 The conqueror comes to tea: Japan under MacArthur by John La Cerda:
1949's The old breed: a history of the First Marine Division in World War II by George McMillan tells us the phrase became so popular it was used as a code for a fight or action:
Finally, the first I found actually using shit is also from WWII in The Naked and the Dead, the 1948 novel by Norman Mailer:
When the shit hits the fan
Messy and exciting consequences brought about by a previously secret situation becoming public.
This expression alludes to the unmissable effects of shit being thrown into an electric fan. It appears to have originated in the 1930s. I can't say better than 'appears' as the earliest citation of it that I can find is in the 1967 edition of Eric Partridge's A Dictionary of Slang and Unconventional English:
"Wait till the major hears that! Then the shit'll hit the fan!"
Partridge lists the phrase as Canadian, circa 1930, but as he gives no supporting evidence we have to go by the 1967 date, although it is undoubtedly earlier.
Other, more polite, forms of the phrase, involving eggs, pie, soup and 'stuff', can certainly be dated from the USA the 1940s. For example, Max Chennault's Up Sun, 1945:
"Sounds like the stuff was about to hit the fan."
The Fresno Bee Republican, May 1948, reported on a psychiatrists' convention, under the heading See How Brain Boys Also Run Wild:
"However, once that opening point was settled, the psychiatrists entered wholly in the business of the convention, which culminated, of course, in the selection of officers for the coming year. And that, as the saying goes, was when the soup hit the fan."
The other versions followed soon afterwards.
- shit hits the fan When things get chaotic or uncontrolable, shit has hit the fan. 3,000 people were waiting for the movie 7 hours before it opened. When it was announced that the movie would not be showing, 50 chimps on motorcycles parachuted down from the sky. The monkeys pulled out assorted automatic assault weapons and then the shit hit the fan.
Don't try this at home they say !!!
In proper sense, I believe you can try to model the physics of the solids: the fan is a device that, by its very design is meant to move air. But it can be anything else.
In the general case of some dirty low cohesion matter it will be spread all around. And then the clean up operation is probably going to be a lengthy and unpleasant task. You don't want to be the one who will have to clean the mess.
In figurative sense, when the shit hits the fan means that there is trouble ahead and that it will be very messy.