`Raining cats and dogs' is one of those English idioms that are taught to all students of English as a foreign language, but are never uttered by native English speakers. Its derivation, however, is a mystery.

Has anyone ever really referred to an obstinacy of buffaloes, or a gam of whales, or a bloat of hippopotamuses, or an exaltation of larks? Or are these collective nouns just part of the idiomatic vocabulary taught to foreigners struggling with our language so that we can all have a little laugh when they come out with them?

"Raining cats and dogs" is another one. No true Englishman ever says that. They say "it's bucketing down" or "it's pissing down". So where did the small domestic animals come from?

According to Brewer's Dictionary of Phrase and Fable: "In northern mythology, the cat is supposed to have great influence on the weather." Witches that rode upon storms were said to assume the form of cats, so the cat symbolises the downpour of rain. The dog, says Brewer, "is a signal of wind, like the wolf, both which animals were attendants of Odin, the storm-god." So a rain of cats and dogs is heavy rain with wind.

Graeme Donald's Dictionary of Modern Phrase suggests that there is a connection with the behaviour of the animals being viewed as weather portents. "Should a cat indulge in excessive self-cleaning it heralded protracted rainfall or if a dog kept rolling on the ground then a thunderstorm was close." He also mentions the "lofty theories" linking the phrase to the Greek catadulpa, a waterfall, or kata and doxein meaning "full" and "receptacle" respectively, but points out that 17th-century examples of "raining dogs and polecats" would suggest that the Greek derivation is fanciful.

"Of many explanations," BA Phythian tells us in his Concise Dictionary of Phrase and Fable, "the most popular is that cats and dogs used to drown as a result of heavy rainfall on medieval towns that had no street drainage. Some commentators add that superstitious folk may have assumed that the dead animals fell from the sky." He prefers, however, to see the expression as an extension of an older and more general saying, "with dog and cat", suggesting a disaster.

The man to blame for bringing the phrase into popular use may well be Jonathan Swift, who included it in his Complete Collection of Polite and Ingenious Conversation (1738), but it is not clear whether he cited the expression as one genuinely used, or was making it up as a parody of the sort of nonsense spoken by the chattering classes.

The Oxford English Dictionary credits Swift's as the earliest citation of the expression, but also mentions a 1652 sighting of raining "dogs and polecats". Shelley, in 1819, wrote "raining cats and dogs" in a letter to a friend; Thackeray, writing in Scribner's Magazine in 1849, referred to "pouring with rain ... and the most dismal ... cat and dog day.

The more graphic expression, to rain pitchforks, has never really caught on. Apparently it began with D Humphreys, who wrote in 1815: "I'll be even with you, if it rains pitchforks, tines downwards." Oddly enough, the French may say "Il pleut des hallebardes" (raining halberds) while the Germans have "Es regnet Heugabeln" (pitchforks, again).