(originally posted March 29, 2006)

Definition: It's raining very hard. It's a huge downpour.

Example: Can you believe this weather? One moment it's sunny, and next moment it's raining cats and dogs!

Origin: There are several stories of how this phrase got started. Some say it's related to tornadoes, how it can carry away anything and drop them, but why just cats and dogs?

Another tale says that the expression was originally "cats and ducks", but later was corrupted to "cats and dogs". Cats are mentioned for relation to witchcraft, and ducks for relation to water.

Yet another tale says that in old English houses, the roof is low enough that cats and dogs can jump on there, but in a heavy downpour they'd be washed off, and if you're inside it'd look like it's raining cats and dogs.

Warning: Do NOT be cute and try to subsitute other animals, or even specific breeds.
It's NOT funny when you say "it's raining calicos and dobermans." (Well, maybe it is sort of funny that way...)