I'm getting blazed and laughing at the way they overdoing shit
I'm just a old soul sticking to a newer script
I guess I got to prove this shit, I'm truly too legit to quit

from Earl Sweatshirt (Ft. Domo Genesis) – Knight Lyrics on Rap Genius

Meaning

While too many of today’s tryhard rappers turn to absurd gimmicks rather than actual talent to gain attention, Domo lights one and laughs at their efforts.

He’s got the same love and passion for rap that used to be present in rap’s golden age, but he’s still able to deliver fresh material.

Because he doesn’t fit the mold of most of today’s rappers, he turns to his lyrical ability to gain attention, and being in the shadow of fellow Odd Future members Tyler the Creator, and Earl Sweatshirt, he’s got something to prove to the critics. He’s pretty damn good and he knows it. He’s too legit to quit.

The “truly too legit to quit” part could very well be referring to MC Hammers hit song “2 Legit 2 Quit” from the early nineties, further referring back to Domo saying he’s an old soul.

It could also be a Hot Rod movie reference.

Rod Kimble: I used to be legit. I was too legit. I was too legit to quit. but now I’m not legit. I’m unlegit. And for that reason, I must quit.

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