Definition & Meaning of "rain cats and dogs"

What is the origin of the idiom "rain cats and dogs" and when to use it?

The origin of the idiom "rain cats and dogs" may have derived from the Old English word "catadupe," meaning a waterfall or cataract, which later evolved into "cat-and-dog" rain. This idiomatic expression is used to describe heavy or torrential rainfall. It implies that the rain is falling with great intensity, often accompanied by a loud or forceful sound.