To rain very hard

Thank goodness the kids didn't go out on their bikes, because by mid afternoon it started raining cats and dogs and all hell broke loose

Per fortuna i bambini non sono usciti in bicicletta, perché a metà pomeriggio si sono aperte le cateratte del cielo ed è venuta giù l'ira di Dio

In the middle of the picnic it started to rain cats and dogs, and everybody got soaking wet

Nel bel mezzo del picnic è cominciato a piovere a catinelle e tutti si sono bagnati come pulcini

I'm not going out in that storm. It's coming down in buckets

Non esco certo con questo tempaccio. Sta piovendo a dirotto e sta venendo giù l'ira di Dio

This expression's origin is not clear. Some think it goes back to two centuries ago, when junk and wastes of all sorts - including dead cats and dogs - were simply tossed into the streets, where they accumulated in a gutter along the center, called the kennel. A really violent rain would then dislodge the dead animals from their kennel and sweep them along the street. Others believe it comes from Northern European mythology, where cats were supposed to have great influence on weather and dogs were the symbol of wind: hence cats represent heavy rain and dogs strong wind gusts. A different explanation is that thunder and lightning suggest a cat-and-dog fight