means the reddish-brown flaky coating that forms on iron and steel when they react with oxygen and moisture, slowly eating the metal away.

from From Old English 'rust,' tracing back to a Proto-Germanic root (shared with German 'Rost' and Dutch 'roest') and ultimately to an ancient Indo-European color-word for red — the same deep root that gives us 'ruddy' and 'red.' So the name is, fittingly, just the color the metal turns as it dies.