It's raining cats and dogs

The expression means that it's raining very heavily. It's an interesting old English phrase that no one knows the origin of it.

When it is raining very heavily, we may also use the following idiomatic expressions:
• It's pouring down
• It's pouring from the heavens
• It's bucketing down
• It's flooding down
• It's pissing down
• It's peeing down very colloquial