It’s raining cats and dogs. A common enough phrase.
Makes not the first lick of sense, though. So why do we say it?
Well, there are several folk etymologies floating around, including these from the Library of Congress website:
Odin, the Norse god of storms, was often pictured with dogs and wolves, which were symbols of wind. Witches, who supposedly rode their brooms during storms, were often pictured with black cats, which became signs of heavy rain for sailors. Therefore, “raining cats and dogs” may refer to a storm with wind (dogs) and heavy rain (cats).
“Cats and dogs” may come from the Greek expression cata doxa, which means “contrary to experience or belief.” If it is raining cats and dogs, it is raining unusually or unbelievably hard.
“Cats and dogs” may be a perversion of the now obsolete word catadupe. In old English, catadupe meant a cataract or waterfall. A version of catadupe existed in many old languages. In Latin, for example, catadupa was borrowed from the classical Greek κατάδουποι, which referred to the cataracts of the Nile River. So, to say it’s raining “cats and dogs” might be to say it’s raining waterfalls.
[Another] theory stated that cats and dogs used to cuddle into thatch roofs during storms and then be washed out during heavy rains.
All of that is pure speculation, however, and there’s no evidence that any of it is even remotely accurate.
As it turns out, the phrase didn’t start out as “cats and dogs.” It can be traced back to around the year 1630, in the dialog of a comedic play by Richard Brome entitled The City Wit: or, the woman wears the Breeches, in which a pretentious character translates a passage in Latin which he has no idea how to read. He renders the string of words “regnabitque dogmata polla sophon” as “it shall raine dogs and polecats and so forth.” In context, it’s a great laugh line.
That line apparently caught on as a catchphrase, which was shortened to “rain dogs and cats.” About 20 years after The City Wit, it crops up in the poem Upon a Cloke lent him by Mr. I. Ridsley by Henry Vaughan:
The Pedlars of our age have business yet,
And gladly would against the Fayr-day fit
Themselves with such a Roofe, that can secure
Their Wares from Dogs and Cats rain’d in showre.
A decade later, in 1661, the phrase appears in Flatman and Phillips’ Don Juan Lamberto: or, a comical history of the late times, in a line by a character named Señor Vane:
I am right glad to see you Sr. Lambert, though not so glad to see you here, however it is better to be here than in the open Fields, where there is no shelter against the Rain, nor any other kind of storm that should happen, for here we have Houses over our heads, so that if it should rain Dogs and Cats we could have no harm.
Another decade later, in 1672, it is used by Maurice Atkins in his Catalpus, or, Aeneas, his descent to hell a mock poem in imitation of the sixth book of Virgil’s Aeneis, in English burlesque:
Neither had he flincht a foot, had fates
Made it rain down dogs and cats;
Though old was body and decrepit,
Yet heart was whole and nought could break it.
Shortly thereafter, in 1678, John Phillips again uses the term in his “burlesque verse” Maronides, parodying the same work by Virgil:
Under the branches, wot ye well,
When it rains Dogs and Cats in Hell,
The shelder’d Centaurs roar and yell;
Mounted on monkeys, with their tayls
As closely shav’d as back of nayls.
The phrase “rain cats and dogs” with the order of the animals reversed first appears some sixty years later, in 1738, in the Complete Collection of Genteel and Ingenious Conversation, a satire by Jonathan Swift lampooning the speech habits of the upper class.
– Lady Smart. Well; but, Sir John, when may we hope to see you again in London?
— Sir John Linger. Why, Madam, not till the Ducks have eat up the Dirt; as the Children say.
— Mr. Neverout. Come, Sir John; I foresee it will rain terribly.
— Lady Smart. Come, Sir John, do nothing rashly; let us drink first.
— Lord Sparkish. I know Sir John will go, tho’ he was sure it would rain Cats and Dogs.
Although there are gaps of many years between these citations, they all share the common thread of being humorous writings, and the progression strongly suggests that the phrase started out as a joke, caught on as a popular humorous phrase, and eventually morphed into a cliché as people forgot where it came from. The transformations it takes during this nearly century-long arc are consistent with the term being spread by popular usage, and Swift’s lampooning of the phrase as a feature of upper class speech is particularly supportive of a literary origin in the theater and satiric verse rather than as a saying among common folk.
So there you have it. We say “raining cats and dogs” because the British theater-going class four centuries ago thought a joke about a guy pretending to know Latin was side-splittingly hilarious.