As often happens with idioms, they have an obscure etymology. We are so curious about language that we decided to investigate a bit and discovered a couple of interesting things.

Meaning

“To rain cats and dogs” simply means “to rain very hard”.

Origin and Etymology

The etymology of “rain cats and dogs” is unknown. Etymologists came up with a variety of creative (even funny) explanations. Here are some of the most popular theories:

  • According to the mythologic one: dogs and wolves were Odin’s (the god of storms) sacred animals and symbolized the wind, while witches used to ride their brooms in the rain bringing over their cats, symbolizing the rain. Therefore, “raining cats and dogs” would refer to a storm (with wind and rain). Would you seriously buy this? I don't.
  • “Cats and dogs” could be a corruption of the the French word “catadoupe”, meaning waterfall. In this case “raining cats and dogs” would mean something like “raining like a waterfall”. Less funny than the previous, but equally untrustworthy.
  • Another theory stated that the idiom refers to the fact that cats and dogs were washed from thatched roofs during heavy rains. If you believe this theory you would have also to accept that dogs lived on top of thatched roofs. I don't think so.
  • According to the Oxford English Dictionary the first mention of the idiom dates to 1710 and is credited to Jonathan Swift. In his satirical poem 'A Description of a City Shower' he describes how, as a consecuence of a heavy rain, water-borne cat and dog corpses together with other debris were floating in the streets of London. This definitely sounds more credible.

While writing this post, I eventually came across a painting from George Cruikshank dated 1835 and titled Very Unpleasant Weather, or the old saying verified: “Raining Cats, Dogs, & Pitchforks”.

raining cats and dogs Cruikshank Very Unpleasant Weather old saying verified Raining Cats Dogs Pitchforks idiom

Credit: National Gallery of Victoria, Melbourne - NGV Collection Online.

Ludwig's wrap-up

“Rain cats and dogs” is the typical idiom that is taught to English learners, even if it is a completely outdated in real world English.
Considering the UK’s weather, it is no surprise that English language is plenty of ways to say that it is raining. Here you have a wide selection of alternatives to choose from (click on the link to see how are they used in context):
It’s pouring
It’s chucking it down
It’s pelting down
It’s bucketing
It’s (really) coming down
It's coming down in sheets
It’s bucketing down
It’s chucking it down (very British)
It’s raining buckets
It’s lashing
It’s raining pitchforks (outdated)
It’s pissing down (coarse slang)
It’s streaming

If you know other ways to say raining heavily, please write them down in the comments.

P.S. Irony of fate: as soon as I finished writing this post, it started pouring :(

Next time I'll write about sunny weather :)