To the point you don't suck my dick, then you're gonna get decapitated
Other words, you don't fucking give me head, then I'm have to take it

from Bad Meets Evil – Fast Lane Lyrics on Rap Genius

Meaning

Crazy wordplay here. Decapitation means to get your head cut off. So you can either give Eminem “head” (blowjob) or get your head cut-off.

The decapitation/head taking is also playing on the idea that if she doesn’t give it up willingly he is going to take it as in rape.

There is a third meaning that goes with Royce next lines. They want to be the top of the rap game or the head of the rap game so either you give them the top spot or they will fucking take it by killing the ones on top or the rest or the rest of the game. That goes with Royce saying after that they gonna catch a case or get charged with killing the rest of the rap game.

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