the writing on the wall is an idiom for “imminent doom or misfortune” and for “the future is predetermined”. Also cavemen drew on the walls of their caves.
Luke 8:10
He said, “The knowledge of the secrets of the kingdom of God has been given to you, but to others I speak in parables, so that, ”‘though seeing, they may not see; though hearing, they may not understand.’
The line states that I tried to tell you this would happen (in reference to my boasts of being one of the best rappers) but you couldn’t see it or understand what I was saying.