A good friend you want to have sexual relations with. Usually when you call someone "rounst", you are saying: "Hey, I want to bang you".
Rounst is usually a precursor to hidden physical attraction between two friends.
I saw Kelly yesterday and said "What's up Rounst?", and she got so excited to hear that, that we went straight to my house to fuck. We have been best friends since 3 years old.