I sleep on my back cause it's good for the spine
And coffin rehearsal

from Why? – Fatalist Palmistry Lyrics on Rap Genius

Meaning

When placed in a coffin, one is laid on one’s back — he is sleeping on his back as a ‘rehearsal’ for his death.

This forms a bit of a contradiction to the ‘good for the spine’ part of the line — for one, he is doing something beneficial to his health, and for the other, he is practicing for death.

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