I'm dope but shit, you know that
So watch your fucking tone, Jack
Before I run up in your crib rocking a Casey Jones mask

from Chris Webby – Warm Up (Freeverse) Lyrics on Rap Genius

Meaning

Casey Jones is a vigilante from the Teenage Mutant Ninja Turtles universe that often wears a hockey mask. So if anyone tries to disrespect Webby he will show up to their house to take matters into his own hands.

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