The phrase 'Raining cats and dogs' means 'raining very heavily'. This phrase is at least over 350 years old and has been used, in some form or another, to describe the heavy fall of rain since the 1600s. There are theories that are floating around that try to figure out how the saying may have originated or what it derived from, but ultimately the origins of this expression still remain in question.

There's a comedy called "The City Wit", written in the year 1653 by Richard Brome, that uses a modified version of the phrase. The meaning of it still references the weather:

"It shall raine... Dogs and Polecats."

The modern wording of the phrase comes as early as the year 1738. It is used in A Complete Collection of Genteel and Ingenious Conversation, by Jonathan Swift:

"I know Sir John will go, though he was sure it would rain cats and dogs."

More Info: www.phrases.org.uk