Our search of the Internet found plenty of theories, but no definitive answer. Some seemed to center on ancient Greek. One place said the word cat is derived from the Greek word 'catadupe', meaning "waterfall". That's great, but where's the dog?
Another site had a 17th century British origin, where one writer used the phrase when heavy rains would wash dead animals down the streets.
But on a brighter (and cleaner) note: the most popular explanation seems to stem from Northern European Myths that symbolized cats with rain and dogs with wind. Thus, a heavy rain storm with strong winds were: "it's cats and dogs out there."
Check out more theories here: