1
general::
Phrase(s): rain cats and dogs
Fig. to rain very hard. • It’s raining cats and dogs. Look at it pour! • I’m not going out in that storm. It’s raining cats and dogs.
McGrawhill's American Idioms And Phrasal Verbs
2
general::
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 mid1600s, 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.
American Heritage Idioms