What does "raining cats and dogs" mean — and why is it funny?

informal

Meaning

Raining very heavily.

Where it comes from

Used since the 1600s; one theory is that heavy storms once washed animals through the streets of poorly drained cities, though the playful image is what kept the phrase alive.

Why it is funny

The humor is the cartoon picture it forces into your head. For a moment you genuinely imagine pets falling from the sky. It is an everyday phrase that, taken literally, is completely surreal, and that clash between the dull weather report and the vivid nonsense is the joke.

Used in a sentence

"Take an umbrella — it's raining cats and dogs out there."