1. Any of various powdery or scaly reddish-brown or reddish-yellow hydrated ferric oxides formed on iron and iron-containing materials by low-temperature oxidation in the presence of water. 2. Any of various metallic coatings, especially oxides, formed by corrosion. 3. A stain or coating resembling iron rust. 4. Deterioration, as of ability, resulting from inactivity or neglect. 5. Rust fungus. 6. A plant disease caused by a rust fungus, characterized by reddish or brownish spots on leaves, stems, and other parts. 7. A strong brown. 8. To become corroded. 9. To deteriorate or degenerate through inactivity or neglect. 10. To become the color of rust. 11. To develop a disease caused by a rust fungus. 12. To corrode or subject (a metal) to rust formation. 13. To impair or spoil, as by misuse or inactivity. 14. To color (something) a strong brown.
