Why do we say 'it is raining cats and dogs'?

This phrase became popular several centuries ago at a time when the streets of towns and cities were narrow, filthy and had poor drainage.

Unusually heavy storms produced torrential flooding which drowned large numbers of the half-starved cats and dogs that foraged there. After a downpour was over, people would emerge from their houses to find the corpses of these unfortunate animals, and the more gullible among them believed that the bodies must have fallen from the sky and that it had literally been raining cats and dogs. A description of the impact of a severe city storm, written by Jonathan Swift in 1710, supports this view: 'Now from all parts the swelling kennels flow, and bear their trophies with them as they go… drowned puppies, stinking sprats, all drenched in mud, dead cats, and turnip-tops, come tumbling down the flood." Some classicists prefer a more ancient explanation, suggesting that the phrase is derived from the Greek word for a waterfall: catadupa. If rain fell in torrents – like a waterfall – then the saying 'raining catadupa' could gradually have been converted into 'raining cats and dogs'.