The English-language idiom raining cats and dogs is used to describe particularly heavy rain. It is of unknown etymology. One possible explanation involves the drainage systems on buildings in 17th-century Europe, which were poor and may have disgorged their contents, including the corpses of any animals that had accumulated in them, during heavy showers.
This occurrence is documented in Jonathan Swift’s 1710 poem ‘Description of a City Shower,’ in which he describes: ‘Drowned puppies, stinking sprats, all drenched in mud, Dead cats and turnip-tops come tumbling down the flood.’
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The Daily Omnivore 