,it's raining heavily, It's raining very hard
it's pouring down
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حصلت هذي القصة في النت عن سبب الاسم
The dog, an attendant of the storm king Odin, was a symbol of wind. Cats came to symbolize down-pouring rain, and dogs to symbolize strong gusts of wind. A very heavy storm, therefore, indicated that both cats and dogs were involved. Another explanation is that the phrase came about in early 17th-century London, when cats hunted mice on the rooftops - during a rainstorm, the cats were washed off the roofs and fell on passersby.