Here "rot away" is a metaphor for wasting away and dying. Alternate translation: "you will become very thin and slowly die" (See: [[rc://en/ta/man/jit/figs-metaphor]])
This implies that God will not forgive the sins of these people. Alternate translation: "and I will not forgive your sins" (See: [[rc://en/ta/man/jit/figs-explicit]])