1. yes, supposedly, although I have never heard the term. Yes, it does mean slang.
2.I think so. I'm not familiar with many idioms, so I'm not sure if there is another one which has a similar meaning.
3. deal secretly with someone suspicious.
Hope these help!
Here are some links to idiom dictionaries.
Idioms and phrases
The Idiom Connection
Self-Study Idiom Quizzes (ESL, EFL)