Raining cats and dogs
Raining very heavily.
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, that is, 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.
One supposed origin is that the phrase derives from mythology. Dogs and wolves were attendants to Odin, the god of storms, and sailors associated them with rain. Witches, who often took the form of their familiars - cats, are supposed to have ridden the wind. Well, some evidence would be nice. There doesn't appear to be any to support this notion.
It has also been suggested that cats and dogs were washed from roofs during heavy weather. This is a widely repeated tale. It got a new lease of life with the e-mail message "Life in the 1500s", which began circulating on the Internet in 1999. Here's the relevant part of that:
I'll describe their houses a little. You've heard of thatch roofs, well that's all they were. Thick straw, piled high, with no wood underneath. They were the only place for the little animals to get warm. So all the pets; dogs, cats and other small animals, mice, rats, bugs, all lived in the roof. When it rained it became slippery so sometimes the animals would slip and fall off the roof. Thus the saying, "it's raining cats and dogs."
This is nonsense of course. It hardly needs debunking but, lest there be any doubt, let's do that anyway. In order to believe this tale we would have to accept that dogs lived in thatched roofs, which, of course, they didn't. Even accepting that bizarre idea, for dogs to have slipped off when it rained they would have needed to be sitting on the outside of the thatch - hardly the place an animal would head for as shelter in bad weather.
Another suggestion is that 'raining cats and dogs' comes from a version of the French word 'catadoupe', meaning waterfall. Again, no evidence. If the phrase were just 'raining cats', or even if there also existed a French word 'dogadoupe', we might be going somewhere with this one. As there isn't, let's pass this by.
There's a similar phrase originating from the North of England - 'raining stair-rods'. No one has gone to the effort of speculating that this is from mythic reports of stairs being carried into the air in storms and falling on gullible peasants. It's just a rather expressive phrase giving a graphic impression of heavy rain - as is 'raining cats and dogs'.
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 Tatler magazine. 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’s A 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.