The phrase to rain cats and dogs means to rain very hard—synonyms: to rain stair rods and to rain pitchforks.
—Cf. also the jocular extended form to rain cats and dogs and to hail cabs.

Although B. A. Phythian made an interesting hypothesis as to the origin of this phrase in A Concise Dictionary of Phrase and Fable (1993), to rain cats and dogs is certainly based on a cat-and-dog fight as a metaphor for a storm or hard rain. The image of the traditional enmity between cats and dogs symbolising discord dates back to Elizabethan times. In The Schoole of Abuse, Conteining a plesaunt inuective against Poets, Pipers, Plaiers, Iesters and such like Caterpillers of a Commonwelth (1579), Stephen Gosson (1554-1625), anti-theatrical polemicist and Church of England clergyman, wrote:

He that compareth our [musical] instruments, with those that were vsed in ancient times, shall see them agree like Dogges and Cattes, and meete as iump as Germans lippes.

This image of strife explains the following sentence from an 1849 letter that the British novelist William Makepeace Thackeray (1811-63) wrote to Mrs Brookfield:

Pouring with rain at Park Lodge, and the most dismal, wretched, cat and dog day ever seen.

To rain cats and dogs has the same intensive and derogatory force as the equivalent French phrases Quel temps de chien! (i.e., What a dog’s weather!) and C’est un temps à ne pas mettre un chien dehors! (i.e., It’s not fit to put a dog out!). In Notes and Queries of 9th November 1861, a certain A. De Morgan wrote that the English phrase “seems to be a simple monster of comparison, like ‘blowing great guns’”.

Interestingly, the travel writer Philip Thicknesse (1719-92) seems to confirm this opinion in Observations on the Customs and Manners of the French Nation, in a Series of Letters, in which that Nation is vindicated from the Misrepresentations of some Late Writers (1766):

I told you in my last I would say nothing of Boulogne, but as it blows cats and dogs, as the sailors say, I have nothing to do but write till it abates.

Likewise, A. Smythe Palmer wrote the following in Folk Etymology, a dictionary of verbal corruptions or words perverted in form or meaning, by false derivation or mistaken analogy (1882):

Cats and dogs, to rain:
Chien, in the French phrase une pluie de chien (a heavy shower), has the same depreciatory and intensive force as in bruit de chien [= dog’s noise], querelle de chien [= dog’s quarrel]. Probably, [to rain cats and dogs] is just one of those strong intensive phrases in which the populace delights. In the dialect of the Wallon de Mons, pleuvoi à dik et dak is to rain in torrents (corresponding to a German regnen dick und (? an) dach, “thick on thatch” – cf. risch und rasch, kling und klang, &c.).

 

HISTORY

 

The following passage from the comedy The City Wit. Or, The woman wears the Breeches (probably first performed between 1629 and 1631), by the English playwright Richard Brome (circa 1590-1652), is the first recorded usage of the notion of raining cats and dogs:

– Sarpego: I will now breath [= speak] a most strong and Poeticall execration [= statement of hatred]
                   Against the Universe.
– Bridget: Sir I beseech you—
– Sarpego: From henceforth Erit Fluvius Deucalionis
                 The world shall flow with dunces; Regnabitque, and it shall raine
                 Dogmata Polla Sophon, Dogs and Polecats, and so forth.

Explanations:
The Latin sentence “erit fluvius Deucalionis” means “it will be the flood of Deucalion”, that is, “let the world be destroyed as in Deucalion’s flood”.
In Greek mythology, the virtuous Deucalion, somewhat like Noah in the Christian tradition, is saved, along with his wife Pyrrha, by building a boat when the world is destroyed by a flood. The flood is sent by Zeus after he becomes outraged by the wickedness of human beings.
Whatever translation skills Sarpego possesses begin to desert him in his fury. “The world shall flow with dunces” does not translate “erit fluvius Deucalionis”. Similarly, “regnabitque” means “and he/she/it shall reign” but the verb is being used primarily to cue Sarpego’s mistranslation “and it shall rain”.
“Dogmata polla sophon” means “many are the thoughts of the wise”. The transliterated Greek word for many, polla, is the reason that Sarpego has “it shall rain […] dogs and polecats” rather than, as in the proverbial phrase, cats and dogs.
Although this is the first known instance of the notion of raining cats and dogs, the fact that Brome is playing with the phrase and offering a comic variation on it (it is raining polecats instead of cats) shows that it was already well established.
(source: E. Schafer – Richard Brome Online – ISBN 978-0-9557876-1-4.)

The phrase was also used by the Welsh author Henry Vaughan (1621-95) in the poem titled Upon a Cloke [= cloak] lent him by Mr. I. Ridsley (published in 1651):

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 [= shower].

In Don Juan Lamberto: or, a comical history of the late times (1661), the English poet and miniature painter Thomas Flatman (1635-88) wrote:

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.

Maurice Atkins also used the phrase in Cataplus, or, Æneas, his descent to hell a mock poem in imitation of the sixth book of Virgil’s Æneis, in English burlesque (1672):

Where e’re I went on Land or water
Hee’d make a shift to follow after.
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.

In Maronides; or, Virgil Travesty, Being a New Paraphrase in Burlesque Verse, Upon the Fifth and Sixth Book of Virgil’s Æneids, published in 1678, the English author John Phillips (1631-1706) wrote:

When it rains Dogs and Cats in Hell,
The shelter’d Centaurs roar and yell;
Mounted on Monkeys, with their tayls
As closely shav’d as back of nayls.

The phrase in its present form is first recorded in A Complete Collection of Genteel and Ingenious Conversation, According to the Most Polite Mode and Method Now used At Court, and in the best Companies of England (published in 1738, but written in the first decade of the 18th century), by the Irish satirist, poet and Anglican cleric Jonathan Swift (1667-1745):

– 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.

This book is a satire on the use of clichés: its purported author, Simon Wagstaff, declares “that there is not one single witty phrase in this whole collection which hath not received the stamp and approbation of at least one hundred years”. This is corroborated by the fact that the first known usage, in The City Wit, of the notion of raining cats and dogs dates from around 1630.

So, when we come across the phrase in its present form in Swift’s book, we can be sure that the metaphor had been well known for about a century at least, and that he, or his contemporaries, only did the work of turning it round, perhaps to make it a little more euphonious to modern ears.

The earlier form has long survived; for example, the following is from the Scunthorpe Club Notes and News, published in the Scunthorpe Evening Telegraph (Lincolnshire) of 17^th November 1939:

The week-end tourists had a long trail and discovered some interesting people who were in town that night!
The weather may be black or raining dogs and cats, but they can not hold back the troops from Micks and Pats.

 

FOLK ETYMOLOGIES

 

A popular theory absurdly says that Swift coined to rain cats and dogs in A Complete Collection of Genteel and Ingenious Conversation, on the grounds that he wrote the following lines in A Description of a City Shower, a poem first published in 1710:

Sweepings from Butcher’s 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.

According to this theory establishing a connection between the two texts written by Swift, the origin of to rain cats and dogs is the fact, evoked in A Description of a City Shower, that, in the filthy British streets of that time, 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 made Swift (says this ludicrous theory) coin the phrase to rain cats and dogs, which he quoted in A Complete Collection of Genteel and Ingenious Conversation. (One wonders why he did not coin the phrase to rain sprats and turnips…)

 

According to another theory, in the phrase, the words cats and dogs were originally cat-bolts and dog-bolts, denoting respectively the iron bars for securing a door or gate and the bolts for fastening together pieces of timber.

This theory is based on “in steede of thunderboltes, shooteth nothing but dogboltes or catboltes”, a quotation from Pierces Supererogation: or A New Prayse of the Old Asse (1593), by the English scholar and writer Gabriel Harvey (circa 1552-1631).

Because of the presence of the word thunderbolts in this quotation, this theory says that people compared a shower, or better a hailstorm, to heavy instruments (cat-bolts and dog-bolts) falling on their heads from the sky, with thunderbolt supplying a convenient model for the other two words, and that the second element -bolts was later left out, perhaps because the whole came out too bulky or as a joke (whose humour soon became incomprehensible).

Unfortunately for this theory, the context shows that Gabriel Harvey did not refer to the weather:

I looked either for a fine-witted man, as quicke as quicksilver, that, with a nimble dexterity of lively conceite and exquisite secretaryship, would out-runne mee many hundred miles in the course of his dainty devises; a delicate minion, or some terrible bombarder of tearmes, as wild as wildfire, that, at the first flash of his fury, would leave me thunderstricken upon the ground, or, at the last volley of his outrage, would batter me to dust and ashes. A redoubted adversary! But the trimme silke-worme I looked for (as it were in a proper contempt of common finenesse) prooveth but a silly glow-woorme, and the dreadfull enginer of phrases, in steede of thunderboltes, shooteth nothing but dogboltes and catboltes, and the homeliest boltes of rude folly.

The context shows that cat-bolts and dog-bolts, as well as the homeliest bolts, are only used as disparaging metaphorical terms, in contrast to the metaphorical thunderbolts expected by the author. The metaphor is based on the word bolt in the sense of a discharge of lightning, as in the phrase a bolt from, or out of, the blue, which refers to the unlikelihood of a thunderbolt coming from a clear blue sky. It is therefore probable that the dogs and cats of dog-bolts and cat-bolts only represent the most familiar, the most domestic, “the homeliest” presences that one can think of.

I have exposed several other folk etymologies, in particular in the following articles:
– origin of ‘Indian summer’ and French ‘été sauvage’
– the authentic origin of ‘a pretty kettle of fish’
– The usual explanation of ‘Hobson’s choice’ is fallacious.
– origin of ‘once in a blue moon’
– Kilkenny cats
– to buy a pig in a poke vs. to let the cat out of the bag
– origin of ‘to buttonhole’ (to detain in conversation)
– origin of ‘point-blank’
– between the devil and the deep blue sea
– meaning and origin of ‘the devil to pay’
– origin of ‘to turn a blind eye’.

 

The phrase to rain cats and dogs has sometimes been extended to and pitchforks:

Very unpleasant weather, or the old saying verified “Raining cats, dogs, & pitchforks”!!!
(London: George Humphrey, 27th April 1820), by George Cruikshank (1792-1878).
Image: © The Trustees of the British Museum, released under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International (CC BY-NC-SA 4.0) Licence.