I can eat a pound and shit sixty four quarters
There’s sixty four quarter ounces in a pound. Brown is so used to drug dealing he can figuratively take a pound and shit it out into equal amounts.
Also there are 64 quarter notes in 16 4/4 measures. He’s using this drug metaphor to describe his rapping.
To help improve the meaning of these lyrics, visit "Grown Up" by Danny Brown Lyrics 2 and leave a comment on the lyrics box