Raining cats and dogs means pouring with very heavy rain. It is raining cats and dogs.
Nobody knows where it comes from, and every confident explanation of it is invented. Raining cats and dogs is one of the first idioms learners are taught and one of the least used.
1phraseraining cats and dogsPouring with very heavy rain.
“We waited an hour because it was raining cats and dogs.”
raining cats and dogs is a phrase.
rain cats and dogs unknown, but see examples of early use in Citations.
Etymology from Wiktionary, CC BY-SA
Practised in Idioms