I heard them chatter: "He's probably young but I know that he's down"
Step on his neck as hard as your bullet proof vest

from Kendrick Lamar (Ft. Chad Hugo & Pharrell) – Good Kid Lyrics on Rap Genius

The police disregard his young age and assume that he’s “down” (gang affiliated), probably just because he’s black. They rough him up based on this assumption, in exactly the same way as the gang members in the first verse.

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