Here's why:
* It's a figurative expression related to rain. The idiom doesn't literally mean cats and dogs are falling from the sky. It signifies very heavy rain.
* Dictionaries organize entries based on subject matter. A dictionary would logically group idioms and phrases related to rain under the relevant headword.
However, depending on the dictionary's structure, it could also be found under:
* Idiom: Some dictionaries have a separate section dedicated to idioms.
* Weather: Dictionaries might group weather-related phrases, including idioms.
* Cat: While unlikely, some dictionaries might list it under "cat" due to its presence in the idiom, although this would be unconventional.
Ultimately, the best way to find it is to check the dictionary's organization and search under the most relevant headword.