Raining cats and dogs...
"Raining cats and dogs"
Meaning: Raining very heavily.
This is an interesting phrase in that, although there's no definitive origin, there is a likely derivation.
One supposed origin is that the phrase derives from mythology. Dogs and wolves were attendants to Odin, the god of storms, and sailors associated them with rain. Witches, who often took the form of their familiars - cats, are supposed to have ridden the wind. Well, some evidence would be nice. There doesn't appear to be any to support this notion.
The first appearance of the currently used version is in Jonathan Swift’s A Complete Collection of Polite and Ingenious Conversation in 1738:
"I know Sir John will go, though he was sure it would rain cats and dogs".
The fact that Swift had alluded to the streets flowing with dead cats and dogs some years earlier and now used 'rain cats and dogs' explicitly is good evidence that poor sanitation was the source of the phrase as we now use it.