It's raining cats and dogs!

Prof Sexson asked us the other day to look up the origins of this phrase. I looked it up and found several different opinions but this seems to be the most agreed upon origin.

This is an interesting phrase in that, although there's no definitive origin, there is a likely derivation. Before we get to that, let's get some of the fanciful proposed derivations out of the way.

The phrase isn't related to the well-known antipathy between dogs and cats, which is exemplified in the phrase 'fight like cat and dog'. Nor is the phrase in any sense literal, i.e. it doesn't record an incident where cats and dogs fell from the sky. Small creatures, of the size of frogs or fish, do occasionally get carried skywards in freak weather. Impromptu involuntary flight must also happen to dogs or cats from time to time, but there's no record of groups of them being scooped up in that way and causing this phrase to be coined. Not that we need to study English meteorological records for that - it's plainly implausible.

The much more probable source of 'raining cats and dogs' is the prosaic fact that, in the filthy streets of 17th/18th century England, heavy rain would occasionally carry along dead animals and other debris. The animals didn't fall from the sky, but the sight of dead cats and dogs floating by in storms could well have caused the coining of this colourful phrase. Jonathan Swift described such an event in his satirical poem 'A Description of a City Shower', first published in the 1710 collection of the Tatlermagazine. The poem was a denunciation of contemporary London society and its meaning has been much debated. While the poem is metaphorical and doesn't describe a specific flood, it seems that, in describing water-borne animal corpses, Swift was referring to an occurrence that his readers would have been well familiar with:

Now in contiguous Drops the Flood comes down,
Threat'ning with Deluge this devoted Town.
...
Now from all Parts the swelling Kennels flow,
And bear their Trophies with them as they go:
Filth of all Hues and Odours seem to tell
What Street they sail'd from, by their Sight and Smell.
They, as each Torrent drives, with rapid Force,
From Smithfield or St. Pulchre's shape their Course,
And in huge Confluent join'd at Snow-Hill Ridge,
Fall from the Conduit, prone to Holbourn-Bridge.
Sweeping from Butchers Stalls, Dung, Guts, and Blood,
Drown'd Puppies, stinking Sprats, all drench'd in Mud,
Dead Cats and Turnip-Tops come tumbling down the Flood.

We do know that the phrase was in use in a modified form in 1653, when Richard Brome's comedy The City Wit or The Woman Wears the Breeches referred to stormy weather with the line:

"It shall raine... Dogs and Polecats".

Polecats aren't cats as such but the jump between them in linguistic rather than veterinary terms isn't large and it seems clear that Broome's version was essentially the same phrase. The first appearance of the currently used version is in Jonathan Swift’sA Complete Collection of Polite and Ingenious Conversation in 1738:

"I know Sir John will go, though he was sure it would rain cats and dogs".

The fact that Swift had alluded to the streets flowing with dead cats and dogs some years earlier and now used 'rain cats and dogs' explicitly is good evidence that poor sanitation was the source of the phrase as we now use it.