If it is raining cats and dogs it is raining very heavily.
It’s not known where this expression comes from, but it does not mean that cats and dogs are falling from the sky 🙂
Some people think it might be related to the fact that cats and dogs are traditionally thought of as old enemies; and when it’s raining heavily the raindrops are like cats and dogs fighting.
Other people think it’s more likely that this idiom has its origins in the filthy and badly-drained streets of 17th / 18th century England. When it rained heavily all sorts of rubbish, including dead cats and dogs, would have floated down the flooded streets as if they had fallen from the sky with the rain.
It might also have come from the old French word catadupe, meaning waterfall.
Examples of use:
1. It’s raining cats and dogs today so I’m staying indoors.
2. It’s raining cats and dogs outside. I’m soaked!
3. Put your coat on: it’s raining cats and dogs out here.
Image © mira66