The phrase it’s “Raining cats and dogs” is first found in writing in 1651 in a poem called Olor Iscanus by a British poet called Henry Vaughan – “the roof was secured against dogs and cats rained in shower”.

The meaning and origin has been lost in the ages, but there are three main theories that could explain how this saying originated.

Norse Mythology

Odin, the god of storms was always in the good company of dogs and wolves which happened to be the masters of the wind, an important element of good thunderstorms. On the others side, witches were often associated with cats. Cats in general were always associated with influencing bad weather. Black cats in particular were shunned by sailors. Seeing a black cat before sailing meant that their new voyage was going to be accompanied by heavy rain and bad luck. Combining the two, it is reasonable to think that dogs and cats were associated with a stormy, windy, hard rain.

A strange twist of Greek and Latin words

A corruption of a Greek phrase “kata doksa” meaning “Contrary to expectations” could actually be a possible explanation. In English Cataclysm and orthodoxy are exactly the words that the Greek sometimes use even today to describe a heavy waterfall. An unexpected heavy rain “KATa” and DO(G)ka” could be a simple explanations for an unusual twist of words and meaning.

Dead animals being picked up after a storm

Drainage systems were horrible in the 17th century so it was rumored that dead animals were the cause of it. Their corpses were often found everywhere stopping proper drainage. Jonathan Swift in 1710 describes : “Drowned puppies, stinking sprats, all drenched in mud, Dead cats and turnip-tops come tumbling down the flood.”

References to know more

Library of Congress
It’s raining cats and dogs! – Idioms by The Free Dictionary
Wikipedia
A nice blog