A literal explination for raining cats and dogs is that during heavy rains in 17-century England some city streets became raging rivers of filth carrying many dead cats and dogs. The first printed use of the phrase does date to the 17th centurey, when English playwright Richard Brome wrote in The City Witt (1652): "It shall rain dogs and polecats." His use of "polecats" certainly suggests a less literal explination , but no better theory has been offered. Other conjectures are the the hyperbole comes from a Greek saying, similar in sound, meaning "an unlikely occurrence," and that the phrase derives from a rare French word, catadoupe ("a waterfall"), which sounds a little like cats and dogs. It could also be that the expression was inspired by the fact that cats and dogs were closely associated witht the rain and wind the Northern mythology, dogs often being pictured as the attendants of Odin the strom god, while cats were believed to cause storms. Similar colloquial expressions include it's raining pirchforks, darning needles, hammer handles, chicken coops, and men.
Geeze, its raining cats and dogs out there!