Neither do etymologists! (The peeps who study the origin of words)
Here's what the Library of Congress has to say about its possible origins:
"Etymologists have suggested a variety of mythological and literal explanations for why people say “it’s raining cats and dogs” to describe a heavy downpour. Here are some of the popular theories:
- Odin, the Norse god of storms, was often pictured with dogs and wolves, which were symbols of wind. Witches, who supposedly rode their brooms during storms, were often pictured with black cats, which became signs of heavy rain for sailors. Therefore, “raining cats and dogs” 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.
- “Cats and dogs” may be a perversion of the now obsolete word catadupe. In old English, catadupe meant a cataract or waterfall. A version of catadupe existed in many old languages.In Latin, for example, catadupa. was borrowed from the classical Greek κατάδουποι, which referred to the cataracts of the Nile River. So, to say it’s raining “cats and dogs” might be to say it’s raining waterfalls.
- A false theory stated that cats and dogs used to cuddle into thatch roofs during storms and then be washed out during heavy rains. However, a properly maintained thatch roof is naturally water resistant and slanted to allow water to run off. In order to slip off the roof, the animals would have to be lying on the outside—an unlikely place for an animal to seek shelter during a storm. "
Our fave is cata doxa - what's yours?
source: Library of Congress online, http://www.loc.gov/rr/scitech/mysteries/rainingcats.html