rain cats and dogs
Also, rain buckets. Rain very heavily, as in It was raining cats and dogs so I couldn't walk to the store, or It's been raining buckets all day. The precise allusion in the first term, which dates from the mid-1600s, has been lost, but it probably refers to gutters overflowing with debris that included sewage, garbage, and dead animals. Richard Brome used a version of this idiom in his play The City Wit (c. 1652), where a character pretending a knowledge of Latin translates wholly by ear, “Regna bitque/and it shall rain, Dogmata Polla Sophon/dogs and polecats and so forth.” The variant presumably alludes to rain heavy enough to fill pails.