refering to a heavy downfall of rain, origins of this saying :-

came about because in the 1700's when towns and cities had narrow streets, and drainage was poor. Heavy storms resulted in flooding which drowned the weaker cats and dogs. After the storm was over, people would find the corpses littering the streets - as if it had literally been raining cats and dogs.

alternatively :-

The cat is supposed to have influence over the weather according to northern mythology. Witches assumed the form of a cat, and were said to ride on storms. Odin, the storm god was attended by a dog & wolf, both being signs of the wind. Therefore the cat could be taken as a symbol of rain, and dog of the accompanying winds. 


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