How did the phrase raining cats and dogs come about?
The most common one says that in olden times, homes had thatched roofs in which domestic animals such as cats and dogs would like to hide. In heavy rain, the animals would either be washed out of the thatch, or rapidly abandon it for better shelter, so it would seem to be raining cats and dogs
It also may refer to a storm with wind (dogs) and heavy rain (cats). “Cats and dogs” may come from the Greek expression cata doxa, which means “contrary to experience or belief.” If it is raining cats and dogs, it is raining unusually or unbelievably hard
So now when you hear that saying, you now know how it may have started.