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When the shit hits the fan

Meaning

Messy and exciting consequences brought about by a previously secret situation becoming public.

Origin

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.