Simple genius, go hard and spit bits of semen
It’s a metaphor for a penis — “go hard,” meaning an erection and a common term in rap for rapping well and “spit bits of semen,” ie the hard dick is spitting semen.
This could also be a play on words. Semen sounds like cement and cement is hard. Earl could be saying that he spits hard like cement. The next line “So when the street split” supports this double-meaning as well.
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