Backpack rap crap, yap-yap, yackety-yack
And at the exact same time
I attempt these lyrical acrobat stunts while I'm practicing that

from Eminem – Rap God Lyrics on Rap Genius

Meaning

Em raps with such eloquence that he can make non-rhyming words sound like they rhyme. This tricky pronunciation is referred to here as “lyrical acrobat stunts”.

http://www.youtube.com/watch?v=_kQBVneC30o

“Backpack Rap” is a term used to describe rap artists who go against the mainstream. Yackity-yak is noisy talk: talking without saying anything. “Yackity-yack backpack” rappers might be similar to the “lyrical miracle” rappers he has mentioned derisively in the past

The line may be a reference to the song “Yakety Yak” by The Coasters. In the song, “yackety yack” is followed by “don’t talk back,” which Em may be silently implying here.

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