If you look out the window and see a heavy rainstorm, you should not be surprised if you hear an English speaker say, “It’s raining cats and dogs!” This expression is used to describe heavy rain. Some other ways to say this include a ‘torrential rain’, a ‘downpour’, or a ‘cloudburst’.
You may think that this phrase has its basis in the well-known conflict between canines and felines, similar to the saying “to fight like cats and dogs,” but this expression actually has a surprising medieval origin.
Many historians believe that the expression ‘it’s raining cats and dogs’ emerged in London during the Great Plague of 1665. The bubonic plague was a deadly disease that spread via fleas. These tiny blood-sucking insects would bite infected rats and then transmit the illness to humans. The plague – also known as the ‘black death’ – had a 60% mortality rate and killed many people across Europe. Over 100,000 people in London alone lost their lives.
So, where do cats, dogs and the weather come into this historical disease? Well, there are two conflicting theories.
The first is that humans were not the only victims of the plague. Cats and dogs also contracted and transmitted this horrific disease, spreading it via fleas in their fur. Some say that so many animals died during the Great Plague that their bodies lined the gutters. So when a heavy rain would occur their corpses would be swept down the street in a torrent.
The second theory is that in the 17th century, most people lived in simple houses with thatched roofs, and cats and dogs (not to mention birds and other livestock) would sleep in the soft thatch. When the rains were very heavy, the animals would be dislodged from their resting places and fall from the sky.
The dark history of this expression is mostly unknown to even native English speakers, but people use the expression all the time. If you spend any time in a rainy city, you are sure to hear English speakers say, “Grab your umbrella – it’s raining cats and dogs out there!”
from Deep English » Blog http://deepenglish.com/2016/04/its-raining-cats-and-dogs/