Knock knock, who could it be? Niggas know I got it on lock
Yawk, yawk - bitch rappers better move ‘fore them niggas get dropped

from Childish Gambino (Ft. Ab-Soul & ScHoolboy Q) – Unnecessary Lyrics on Rap Genius

Meaning

Schoolboy Q is one of the hottest MCs in the game right now. You oughta know to get out of his way or he will drop you (remove you from the rap game).

Yawk is onomatopoeia for gunshots. .

When he says, “Knock knock, who could it be? Niggas know I got it on lock.” They know he has the game on lock with his flow, so they knocking, as if they can’t even come through without permission.

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