Its origin may be related to the underground drainage system used in the 17th century,
when the drainage system was very primitive and the drainage capacity was extremely limited.
When it rains heavily, sewage flows from underground drains. With the outflow of sewage not only garbage,
all kinds of filth, and sometimes even dead cats and dogs and other small animal corpses spread out with the sewage.
Dogs and cats became associated with heavy rain, and people coined the expression rain cats and dogs.