noun

1. the reddish-brown or reddish-yellow coating formed on iron or steel by oxidation, as during exposure to air and moisture: it consists mainly of ferric oxide, FeO, and ferric hydroxide, Fe(OH)
2. any coating or film formed on any other metal by oxidation or corrosion
3. any stain or formation resembling iron rust
4. any habit, influence, growth, etc. injurious to usefulness, to the mind or character, etc.
5. disuse of mental or moral powers; inactivity; idleness
6. the color of iron rust; reddish brown or reddish yellow
7. Bot.
   1. any of various plant diseases characterized by a spotted reddish or brownish discoloration of stems and leaves
   2. any of an order (Uredinales) of basidiomycetous fungi causing rust
      in full rust fungus

Etymology: ME < OE, akin to Ger rost < IE base *reudh-, red

intransitive verb, transitive verb

1. to affect or be affected by a rust fungus
2. to become or cause to be coated with rust, as iron
3. to deteriorate or spoil, as through disuse a mind that has rusted
4. to become or make rust-colored