So for people whose first language is not English, this phrase means “it is raining hard” or ” it is raining very heavily”.
This phrase is called an “idiom”. An idiom is a phrase that cannot be understood from the individual meaning of the words used.
The best source for the answer to this1 says that the idiom “raining cats and dogs” is most likely to have come from a poem by Jonathan Swift, called “A Description Of A City Shower“, which fortunately and thanks to our dear friend Google I can publish here in full as it is in the public domain.
The poem was probably published reasonably widely but this example comes from: The Works of Jonathan Swift: Miscellaneous poems By Jonathan Swift, Sir Walter Scott published in 1814. The poem, however, dates to 1710 and describes London, England, of that era which was a pretty dirty and uncomfortable place by today’s standards by all accounts. It was first printed in the Tatler. Here it is:
The last two lines (that are braketed) refer to “Drown’d” puppies and “Dead cats” that were washed down with the floods. This is not describing dogs and cats falling from the sky but there is a direct link between heavy rain that caused flooding and these companion animals.
And in 1774 and in earlier and later books, the idiom is clearly stated in its modern form. Here is one example from: The works of Dr Jonathan Swift, Dean of St Patrick’s, Dublin. In thirteen … – Page 161
The phase “it would rain cats and dogs”, is mentioned 8 lines from the bottom of this piece of text. These images link back to the books on Google’s massive and impressive database. There is no doubt that these almost ancient examples of the use of this phrase are the correct origin for it.
One last quite important point. People in England very rarely if ever use the phrase today (2010) in my experience. We would use more modern phrases such as “It p*ss*d down”!
Raining Cats And Dogs — Reference:
Photo by AlmazUK – see original on Flickr (this is a cropped version allowed under the license).