And y'all ain't gotta say it, just think it
Cause I know that you know that I know that you know that I know that you know that I'M A GOAT

from Eminem – G.O.A.T. Lyrics on Rap Genius

Meaning

Eminem knows that everybody knows that he is the best, but not everyone says it.
Here he is probably making reference to the They Don’t Know That We Know They Know We Know joke from Friends Tv Series:

-Rachel: I can not believe he would do that to Mon—Whoa! Joey, do they know that we know?
-Joey: No.
-Rachel: Joey!
-Joey: They know you know.
-Rachel: Ugh, I knew it! Oh I cannot believe those two!
-Phoebe: God, they thought they can mess with us! They’re trying to mess with us?! They don’t know that we know they know we know! Joey, you can’t say anything!
-Joey: I couldn’t even if I wanted to.

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