1. Norse Mythology: This theory suggests the phrase originated from the Norse god Thor's chariot being pulled by two goats, one black and one white. When the goats shook themselves off after a long journey, rain would fall heavily. This might be linked to the phrase "raining cats and dogs," as the animals associated with Thor were "cats" (a term for goats in Old Norse) and "dogs."
2. Old English: This theory suggests the phrase came from the Old English word "cat" which referred to a type of small boat. "Dogs" in this context could refer to smaller, clumsier boats. Heavy rain could cause these boats to be tossed around violently, resembling cats and dogs fighting.
3. 16th Century London: This theory suggests that stray cats and dogs were common in London, and in times of heavy rain, they would often seek shelter together in doorways and other sheltered areas. This "clamor" of animals could have been linked to the heavy rainfall.
4. Biblical Reference: Some suggest the phrase originates from the biblical story of Noah's Ark, where animals of all types are brought aboard during a great flood. This is a less likely origin as the phrase wasn't recorded in any historical texts until the 18th century.
5. Simple Rhyming: This theory suggests the phrase is simply a result of two rhyming words, and the meaning arose out of this rhyming pair. While this may be true, it doesn't explain the connection to heavy rain.
Ultimately, the exact origin of the idiom "raining cats and dogs" remains unclear. It's likely a combination of several factors, including folklore, language evolution, and simple rhyming, that led to its creation and enduring popularity.