Shook at these, unmarked vans
Parked in the dark -- NARC's, where's your heart?

from Nas – N.Y. State of Mind Pt. II Lyrics on Rap Genius

Unmarked police cars, operated by plain clothed police officers, usually monitoring individuals on a tip-off, have the potential to spring surprises on unsuspecting criminals due to their ambiguous nature, which is what makes Nas so paranoid. He sees such dishonest behaviour, if you will, as immoral, and makes this clear to narcotics officers, those that enforce drug laws

Also, possible wordplay on the word ‘narks’. (which means “to agitate or annoy”)

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