rust

Etymology 1

From Middle English rust, rost, roust, from Old English rust, rūst (“rust”), from Proto-West Germanic *rust, from Proto-Germanic *rustaz (“rust”), from Proto-Indo-European *rudʰso- (“red”), from Proto-Indo-European *h₁rewdʰ- (“red”). Cognate with Scots roust (“rust”), Saterland Frisian rust (“rust”), West Frisian roast (“rust”), Dutch roest (“rust”), German Rost (“rust”), Danish rust (“rust”), Swedish rost (“rust”), Norwegian rust, ryst (“rust”). Related to red.

noun

1. The deteriorated state of iron or steel as a result of moisture and oxidation.
   The rust on my bicycle chain made cycling to work very dangerous.
2. A similar substance based on another metal.
   copper rust
3. (color) A reddish-brown color.
   rust:
4. A disease of plants caused by a reddish-brown fungus.
5. (philately) Damage caused to stamps and album pages by a fungal infection.

Etymology 2

From Middle English rusten, from the noun (see above).

verb

1. (intransitive) To oxidize, especially of iron or steel.
   The patio furniture had rusted in the wind-driven spray.
   Your gold and silver have rusted, and their rust will be evidence against you and will eat your flesh like fire. 1946, International Council of Religious Education, The New Covenant, Commonly Called the New Testament of Our Lord and Savior Jesus Christ: Revised Standard Version, James 5:3, page 490
2. (transitive) To cause to oxidize.
   The wind-driven spray had thoroughly rusted the patio furniture.
3. (intransitive) To be affected with the parasitic fungus called rust.
   I am sorry to say that, contrary to the majority, I have to report that the forage rusted rather badly. January 3, 1902, “Mapstone Oats: Further Experiences”, in The Agricultural Journal and Mining Record, volume 4, number 22, page 688
4. (transitive, intransitive, figurative) To (cause to) degenerate in idleness; to make or become dull or impaired by inaction.
   Must I rust in Egypt? never more / Appear in arms, and be the chief of Greece? 1692, John Dryden, Cleomenes, the Spartan Hero, a Tragedy
5. (intransitive) Of a black cat or its fur, to turn rust-coloured following long periods of exposure to sunlight.
   It's very common for black cats to rust during the summer.