It's raining cats and dogs

Origin

Imagine the narrow, unpaved streets of 17th-century London, already slick with mud and refuse. When a violent storm broke, the deluge was so intense that it would flood the gutters, sweeping away everything in its path. Unfortunately, this often included the carcasses of stray animals—cats, dogs, and rats—that had drowned or died in the city's unsanitary conditions. As the torrent raged, these unfortunate creatures would be washed through the streets and sewers, sometimes even appearing to fall from the overwhelmed drainage systems or be carried along by the rushing water, giving the grim impression that the very sky was depositing 'cats and dogs' upon the horrified city below. The phrase thus became a vivid, if gruesome, description of an exceptionally heavy downpour.