So now you're rollin wit us
Nas explains that once you’ve joined the Firm, you must defend their cause, possibly playing on the fact that their group’s name is “The Firm”. “Co-defendants” specifically refers to being a defendant in court; that is, once you’re with the Firm, if shit goes down, you’ll all be facing it together. If someone associated with them wasn’t a co-defendant in that situation, it would probably mean they turned state’s witness or in some way betrayed the rest of the Firm, so that can’t happen.
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