Raining Cats and Dogs
Meaning: When it is raining cats and dogs, it is raining really hard.
Origin: This is the interesting part. In the old days, Norsemen (read: vikings) though that the wind was Odin's dog, chasing a cat that took the form of the rain. So whenever it rained really hard, they would say it was raining cats and dogs. This is one of my favorite phrase origins. (: