Photo credit: Nuit Blanche / Arev Manukian
This is one handy phrase to know if you’re heading for the UK, if you want to put into words this nature phenomenon.
This, as I’m pretty sure you’ve guessed, means pretty heavy rain, a shower, if you like.
Armed with this new expression you could say something like:
– No, I’m not going to the gym, it’s bucketing down out there!
– Did you cycle all the way to work? It was bucketing down!
Confessions of the Linguistic Spy 
What about “raining cats and dogs”? It is not quite as posh, I suppose?
Apparently, ‘cats and dogs’ were already out of date by the time I was in school, which was a while ago 🙂