This describes a heavy cloudburst and there are different theories as to how the expression came to be, the first of which is from Norse mythology.

The cat was supposed to have great influence on the weather and witches that rode on storms were said to assume the form of cats. The dog and wolf, meanwhile, were a symbol of wind, and attended Odin, the Norse storm god.

Raining cats and dogs can be divided into two parts – the cat symbolising the pouring rain and the dog symbolising the strong gusts of wind which accompany a rainstorm.

The most vivid explanation of this picturesque expression suggests that in bygone centuries, the drainage in the streets was so bad that during storms, stray cats and dogs were drowned in the flood that ensued. When the water subsided, their carcasses littered the streets.

Jonathan Swift gives us a picture of what it was like in his Description of a City Shower, penned in 1710:

“Drown’d puppies, stinking sprats, all drench’d in mud, Dead cats and turnip tops, come tumbling down the flood.”

The first written record of the phrase as we know it came in Jonathan Swift’s Polite Conversation (1738) which means Swift may be able to take the credit for coining the expression. Some would contest that origin, because dramatist Richard Brome used the expression in a slightly different form way back in 1635. Brome’s version was “It shall rain dogs and polecats.”