rain cats and dogs rain very hard.

☞ Despite much speculation, there is no consensus as to the origin of rain cats and dogs. Suggestions range from the supernatural (cats being associated with witches who were credited with raising storms, dogs being attendants upon Odin, the Scandinavian storm god) to the down-to-earth (animals in medieval times drowning in flooded streets in times of heavy rain and their bodies being assumed by the credulous to have fallen from the skies). Other versions of the saying are rain pitchforks and, in Britain, rain stair rods, which date from the early 19th century and mid 20th century respectively, and reflect the shaft-like appearance of heavy rain. Rain cats and dogs is first recorded in Jonathan Swift's Polite Conversation (1738).