it's raining cats and dogs

It is raining extremely heavily. We wanted to have a barbecue this weekend, but it's been raining cats and dogs since Friday evening.

If you say it's raining cats and dogs, you mean that it is raining very heavily. You mean she wasn't wearing a coat, even though it was raining cats and dogs? Note: There are several possible explanations for this expression, but none of them can be proved. It may refer to the days when drainage in towns was so poor that cats and dogs sometimes drowned in heavy rainfall. Alternatively, `cats and dogs' could be a corruption or misunderstanding of the Greek word `catadupe', meaning `waterfall', so the expression would originally have been `it's raining like a waterfall'. The origin may also be in Norse mythology, where cats and dogs were sometimes associated with the spirit of the storm.

A heavy downpour. The origin of this expression, which has been around since at least 1700, has been lost. Among the theories are that a raging storm is analogous to fighting cats and dogs; that in North European myth cats supposedly influence weather and dogs symbolize wind; and, currently considered the most likely one, that with the primitive drainage systems in use in the seventeenth century, a heavy rainstorm would cause gutters to overflow with a torrent of debris that included garbage, sewage, and dead animals. Among the earliest appearances in print is a note by Jonathan Swift in 1738: “I know Sir John will go, though he was sure it would rain cats and dogs.”