It is unknown where the English phrase “it’s raining cats and dogs” comes from. But there’s one likely etymology that it is a corruption of the Latin catadupa for “waterfall”. From that same Latin root catadupa, we get the Spanish for waterfall.… catarata.
The c‑t root that begins both catarata and “cats and dogs” does make this etymology plausible. But is it real? I just don’t know.
It does, however, make it easy to remember: a waterfall has the intensity of rain showers even stronger than with “cats and dogs”!
