Well, one rational theory on the origin of the phrase “raining cats and dogs” is that it’s just a comically overblown expression of hyperbole.
However, other theories exist as well. Some believe it literally describes an actuality of the past.
Here’s what we do know for sure. The phrase comes from England, and its first recorded use is by Jonathan Swift in Polite Conversation, written circa 1708 and published thirty years later. An earlier variant, “rain dogs and polecats,” appeared in 1652 in Richard Brome’s City Witt.
One possible explanation reads as such: In London, dwellings were packed closely together and the roofs were used by the stray animals of the city as highways and refuges. Some would die up there, and when a torrential downpour washed debris off the roofs, down would come animal carcasses, too.
Other word experts counter with their own pet explanations (as it were)for example, that rain on metal and wood roofs sounded like a great brouhaha between the animals.
Another theory is that in Norse mythology, cats influenced the weather, and wolves were the companions of Odin, the sky god. Still another cites the archaic French word catdoupe, meaning waterfall or cataract.
In other words, none of the experts really know for sure. And therefore, neither do we.