The phrase "it's raining cats and dogs" means that it is raining very heavily. The exact origin of the phrase is uncertain, with several plausible theories ranging from Norse mythology to medieval living conditions to linguistic evolution. 

One theory suggests that the phrase comes from Norse mythology. The god Odin, who was often associated with storms and rain, was accompanied by dogs and wolves, symbols of wind. Additionally, witches, who were believed to ride their brooms during storms, often had black cats as companions. Thus, a stormy night with heavy rain could be described as involving both cats and dogs. 

Another theory suggests that during heavy rainstorms in medieval Europe, the thatched roofs of houses would get slippery, and animals like cats and dogs, which often rested on the roofs, would slip and fall off. Hence, the heavy rain seemed to cause cats and dogs to fall from the sky. 

Some suggest that the phrase could be a corruption of the Greek expression "cata doxa," which means "contrary to experience" or "unexpected." Over time, this might have morphed into "cats and dogs." 

The phrase might have evolved from an old English expression "catadupe," which means a waterfall or a cataract. The word "catadupe" could have been misinterpreted over time as "cats and dogs." The phrase "raining cats and dogs" appears in literature as early as the 17th century. One of the earliest recorded uses in print is by British playwright Richard Brome in his comedy "The City Wit" (1652), where he writes, "It shall rain dogs and polecats."