Emily Dickinson Lexicon

RUST, n. [Sax. rust; D. roest; G. and Sw. rost; Dan. rust; W. rhwd; Gr. ερυσιβη; probably from its color, and allied to ruddy, red, as L. rubigo is from rubeo. See Ruddy.]

1. The oxyd of a metal; a substance composed of oxygen combined with a metal, and forming a rough coat on its surface. All metals except rhodium, gold and platinum are able to rust.
2. Loss of power by inactivity, as metals lose their brightness and smoothness when not used.
3. Any foul matter contracted; as, rust on corn or salted meat.
4. Foul extraneous matter; as, sacred truths cleared from the rust of human mixtures.
5. A disease in grain, a kind of dust which gathers on the stalks and leaves; in reality, a parasitic fungus or mushroom. Ed. Encyc.