The phrase "raining cats and dogs" is of unknown etymology. A number of improbable folk etymologies have been put forward to explain the phrase, for example:

In 16th century Europe when peasant homes were commonly thatched, the home was constructed in such a manner that animals could crawl into the thatch and find shelter from the elements, and would fall out during heavy rain.

Drainage systems on buildings in 17th century Europe were poor, and may have disgorged their contents during heavy showers, including the corpses of any animals that had accumulated in them. This occurrence is documented in Johnathan Swift's 1710 poem 'Description of a City Shower', in which he describes "Drowned puppies, stinking sprats, all drenched in mud,/Dead cats and turnip-tops come tumbling down the flood."

The Greek word Katadoupoi, referring to the waterfalls on the Nile,[8] sounds similar to "cats and dogs"

The Greek phrase "kata doksa", which means "contrary to expectation" is often applied to heavy rain, but there is no evidence to support the theory that it was borrowed by English speakers.

Raining animals is a relatively rare meteorological phenomenon, although occurrences have been reported from many countries throughout history. One hypothesis that has been furthered to explain this phenomenon is that strong winds travelling over water sometimes pick up debris such as fish or frogs, and carry them for up to several miles. However, this primary aspect of the phenomenon has never been witnessed or scientifically tested.

The animals most likely to drop from the sky in a rainfall are fish and frogs, with birds coming third. Sometimes the animals survive the fall, especially fish, suggesting the animals are dropped shortly after extraction. Several witnesses of raining frogs describe the animals as startled, though healthy, and exhibiting relatively normal behavior shortly after the event.

In some incidents, however, the animals are frozen to death or even completely encased in ice. There are examples where the product of the rain is not intact animals, but shredded body parts. Some cases occur just after storms having strong winds, especially during tornadoes.

However, there have been many unconfirmed cases in which rainfalls of animals have occurred in fair weather and in the absence of strong winds or waterspouts.