They say I'm arrogant, scare opponents when his head swells
Yes, well he got some seratonin in his stem cells

from ForestPunk – Bumrush Lyrics on Rap Genius

Meaning

So this nigga is so full of himself ( I mean, he is explaining his OWN lyrics on rap genius. Cmon. ) that he scares all his opponents, because they are so intimidated by his confidence and outlandish, nigga-I-don’t-give-a-flying-FUCK attitude. Seratonin, a chemical in the brain usually equated with happiness and well being, is flooding his stem cells. He is so on Cloud 9 that he doesn’t care what others think of him. Good for you ForestPunk.

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