‘Raining cats and dogs’ means a very hard rain, or a downpour.
Happily, I haven’t come across any mis-uses of this one – feel free to chip in if you have, my comments section is your comments section – but I hate to think what today’s creatively illiterate might come up with as alternates for it.
There’s a rather nice (but convincingly de-bunked) theory that had it that cats and dogs would shelter in thatched roofs in rainstorms, but if the rain was much more than than a shower, they’d be washed out – hence, heavy rain came to be expressed as ‘raining cats and dogs’. Sadly, as anyone familiar with thatched roofs will realise, thatch is very tight. You won’t fit anything much larger than a beetle in thatch; and while a cat might sun itself on top of a roof, it wouldn’t stay there after more than a couple of drops of rain. Also, the next time I see a dog on a roof of any make will be the first.
A more gruesome, but probably more historically accurate, theory states that due to the poor (read non-existent) drainage prevalent for much of England’s history, smaller domestic animals frequently drowned in very heavy rain, and in the aftermath, their bodies would lie in the roads, giving the appearance of it having literally rained cats and dogs.
There’s also a nice version involving Norse beliefs, cats and dogs having influence over winds and storms, and adoption of the phrase into English. Take your pick.
I’ve seen dates on this one from from the 17th Century to the 19th; I’m inclined to credit the earlier end of the spectrum as thatched roofs became increasingly less common, giving way usually to roof tiles in Britain, the closer to the modern day you get. A gentleman named Jonathan Swift seems to be universally credited with one of its earliest uses in writing, in 1738.
It also seems to be largely a phrase used in British English. I haven’t heard it used in North America, although I suspect it’s been around long enough that most people would understand the basic idea, even if it might sound rather quaintly old-fashioned. My usual favourite source for these posts, Etymonline.com, wasn’t willing to commit to much on this expression.
While this phrase might be tricky to adapt as-is to an SFF world, taking the base idea and running with it would provide some excellent world-building opportunities. Take the mythology angle, and you get the chance to develop a spectrum of creatures with influences over the elements, for example.
What is etymology, and why are you excavating it?
Etymology is like the archeology of a language (definition: the study of the origin of words and the way in which their meanings have changed throughout history).