It's raining cats and dogs, origin

It's probably British, and quite old.
Here is a print published in 1820, titled Very Unpleasant Weather, or the Old Saying verified 'Raining Cats, Dogs and Pitchforks! by artist George Cruikshank

HI, It means. "It is very heavy rain"

It comes from old thatched roofs, where stray animals might be running around. If it was raining hard enough, the roofs would soften enough for the animals, namely cats and dogs, to fall through.

Thanks for your replies everyone.

Anonymous It comes from old thatched roofs, where stray animals might be running around. If it was raining hard enough, the roofs would soften enough for the animals, namely cats and dogs, to fall through.

But most cats hate water so much that they would not be up on a thatched roof in any sort of rain!
My dog, too cowers under some cover inside the house

It means it was raining hard because if you were driving and got hit by a dog it would hit the car HARD

AnonymousIt means it was raining hard because if you were driving and got hit by a dog it would hit the car HARD

Well, the saying originated long before the internal combustion engine was invented.

Dogs and cats used to live under the roof in medieval england, when it poured sometimes the roof collapsed and the cats and dogs would then fall to the ground, hence raining cats and dogs

someone went up in plane with a bucket full of cats and dogs and threw them from the plane onto the villages below. hence the phrase "raining cats and dogs"