Pun on his last name and album title — “Cole”, of course, sounds a lot like “cold”
It is also a pun used by J Cole, with the term “Cole World” either referring to J Cole dominating and taking over the hip-hop scene, or that the world is harsh and cruel (as it sounds like cold — hence why he refers to “aint nothing sunny.”) This demonstrates the evil of mankind in the world.
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Childish uses elements of some styles of music that many consider “white” music and combines them with hip-hop, which many consider “black” music. He takes the two and creates a mixtape (“mixed-tape”), like children whose parents are different races are called “mixed”.