A heavy downpour, rain coming down very quickly and hard.
Example of use:
"There's no way they'll be playing at the park, it's raining cats and dogs out there!"
Interesting fact about It's Raining Cats and Dogs
There is no definite origin of this popular phrase. Its first recorded use was in a collection of poems that was published in 1651. Jonathan Swift introduced the phrase to the mainstream in 1738 when he published a satire on the speaking manner of the upper class called "Complete Collection of Genteel and Ingenious Conversation".
