In many English speaking areas of the world, heavy weather is sometimes described “raining cats and dogs,” suggesting that the rain is extremely heavy and rather unpleasant to be out in. A related saying from some parts of England is “raining stair-rods.” There are a number of explanations for the origins of this phrase, which dates to the 1600s, ranging from the mythological to the macabre. Unfortunately, the macabre explanation is probably the most likely.
On the mythological end of things, some people have suggested that the term comes from the idea that cats and dogs are associated with the weather in some regions of the world. Cats especially have historically been linked with witchcraft and the ability to control the weather. However, this explanation for “it's raining cats and dogs” seems a bit thin when you consider the fact that many cats are not fond of water, and therefore they would have no reason to cause heavy rains, even if they could.
A more whimsical explanation references the thatched roofs of many 16th century homes. This theory suggests that cats and dogs took refuge in the thatch, and when heavy rains came, the animals were washed off the roofs, causing it to look like it was raining cats and dogs. Unfortunately, thatch isn't like a haystack; thatching is made in thick bundles which would not serve to shelter an animal, and no animal would be foolish enough to stand on a roof in the pouring rain.
Some people have also suggested that the term is a reference to the obsolete French term catadoupe, which means waterfall. The related term Old English word catadupe — sometime spelled catadupa — also refers to a waterfall or cataract, usually referring to a shallow, rocky section of the Nile River.
Alas, the real story behind “it's raining cats and dogs” lies in an unpleasant fact of 16th century life. During this era, garbage, including dead animals, was often discarded along roads and streets, as there was no organized garbage collection. As a result, heavy rains would wash garbage into the streets of many towns, filling the streets with an unspeakable collection of feces, dead animals, rotting fruit, and an assortment of other unpleasant materials. A casual observer might have been under the impression that it was raining cats and dogs if he or she had glanced outside after a period of heavy rain, when the streets would have been filled with mud, filth, garbage, and discarded animal carcasses.