This phrase does not have a clear origin but one theory comes from a viral email back in 1999. The email circulated for some time and described how people lived in the 1500s. The email looked something like this:
I'll describe their houses a little. You've heard of thatch roofs, well that's all they were. Thick straw, piled high, with no wood underneath. They were the only place for the little animals to get warm. So all the pets; dogs, cats and other small animals, mice, rats, bugs, all lived in the roof. When it rained it became slippery so sometimes the animals would slip and fall off the roof. Thus the saying, "it's raining cats and dogs."1
|1817 Caricature - Raining cats and dogs|
This email did not explain the true origin of this phrase because dogs did not live in thatched roofs. In addition, the dogs would need to be on the outside of the roof in order to fall off. Dogs were not stupid back in the 1500s. They knew not to sit on top of a roof in a torrential downpour.
When you find yourself walking in a heavy downpour, be thankful that the streets are not littered with dead cats and dogs. If you want to read some positive stories about cats, you can visit my friend Laurel's blog.