My cerebellum, get way more deeper than these other fellas
Cerebellum is a part of the brain responsible for handling notions like motions and coordination.
Big Boi is saying that he thinks much more and is more complex/profound than other rappers (or just people in general) out there.
It also could be that here “cerebellum” being used as a metaphor for the head of his penis (glans penis).
In the long standing tradition of rappers bragging about the size of their sex organs, Big Boi cleverly let’s us know that he can literally get deeper than other men.
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