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Nobody knows for certain where the phrase 'it's raining cats and dogs' comes from, though one possibility is that it originates from the 17th century in England when heavy rains would cause debris of all kinds including animals to wash out of the gutters.
Other languages have similarly expressive idioms for a heavy downpour.

www.loc.gov/rr/scitech/mysteries/rainingcats.html

Etymologists—people who study the origins of words—have suggested a variety of mythological and literal explanations for why people say “it's raining cats and ...

www.phrases.org.uk/meanings/raining-cats-and-dogs.html

The meaning and origin of the phrase 'Raining cats and dogs'. ... Not that we need to study English meteorological records for that - it's plainly implausible.

www.urbandictionary.com/define.php?term=raining%20cats%20and%20dogs

Other conjectures are the the hyperbole comes from a Greek saying, similar in sound, meaning "an unlikely occurrence," and that the phrase derives from a rare  ...

english.stackexchange.com/questions/14273/the-etymology-of-the-phrase-its-raining-cats-and-dogs

I was wondering about the phrase it's raining cats and dogs; I've heard two versions of the meaning of the phrase and I was wondering which ...

www.cracked.com/article_16108_the-bizarre-history-10-common-sayings.html

Apr 7, 2008 ... As in, "It's raining cats and dogs. ... Still others say it has to do with the freak occurrence of frogs or fish falling from the sky (after being swept up ...

dictionary.cambridge.org/us/dictionary/english/it-s-raining-cats-and-dogs

it's raining cats and dogs! definition, meaning, what is it's raining cats and dogs!: something that you say when it is raining heavily. Learn more.

www.worldwidewords.org/qa/qa-rai1.htm

Sep 19, 1998 ... The phrase 'it's raining cats and dogs' is a bit outdated, but what is its ... it is a corrupted version of a rare French word, catadoupe, meaning a ...

www.woofipedia.com/articles/where-does-the-expression-raining-cats-and-dogs-come-from

“It's raining cats and dogs” ... Pet carcasses lining the street made it look as though it had actually rained cats and dogs. ... Why Does My Dog Chase His Tail ?

www.grammar-monster.com/sayings_proverbs/raining_cats_and_dogs.htm

What Is the Origin of the Saying "Raining Cats and Dogs"? The term raining cats and dogs derives from Victorian times when street drainage was so poor that ...