They said everything's gonna be fi-fi-fine
Said everything's gonna be o-o-okay, just don't breathe the air
The stuttering means the words were suspect to begin with. That they shouldn’t have believed it when “The cool” said it would be okay.
The actual meaning I think is that you’ll be fine in the streets and being cool as long as you don’t get too comfortable. Like “The Cool” told them it’s fine, but just don’t breath the air. Something the people falling victim should have found suspicious, but they want to be cool too badly, so they do it anyways.
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