I'm on that shit that you can't smell, baby
A reference to lean, pills, or possibly any number of date rape drugs such as rohypnol an GHB which are undetectable to taste or smell, a scenario suggested by the previous two line.
Or, since the next line tells her to “put down your perfume,” he’s saying, “I’m on that shit [that makes it so that] you can’t smell”—i.e. cocaine, so she doesn’t need to bother with her perfume.
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