«طَحَّانْ مَا يْغَبَّرْ عَلَى كَلَّاسْ»

When a miller's flour dust drifts onto a lime-worker, nothing changes, because the lime-worker is already caked in far heavier dust from his own work. The little flour that lands on him simply vanishes into the coating he already wears, so no one could tell it was there.

Figuratively, a small fault, slight, or trouble makes no impression on someone already marked by something much greater of the same sort. The point is narrow, not a claim of general invulnerability: a lesser blemish disappears against a greater one already present.

Suited to moments when someone tries to taint, blame, or burden a person already bearing a much heavier mark of the same kind. It contrasts degrees of a mark, not the worth of millers or lime-workers as trades.