what is the difference between slang and idiom?
Slang is a language, words...only understood by a specific group of people. You can find prison slang, street slang... Often the words become so common, that they become common knowledge.
- Language peculiar to a group; argot or jargon: thieves' slang.
An idiom is a fixed sentence which is used to express an idea:
he was drunk as a Lord.
I am fit as a fiddle.