"red oxide of iron," Old English rust "rust; moral canker," related to rudu "redness," from Proto-Germanic *rusta- (source also of Frisian rust, Old High German and German rost, Middle Dutch ro(e)st), from PIE *reudh-s-to- (source also of Lithuanian rustas "brownish," rūdėti "to rust;" Latin robigo, Old Church Slavonic ruzda "rust"), from suffixed form of root *reudh- "red, ruddy."

As a plant disease, attested from mid-14c. Rust Belt "decayed urban industrial areas of mid-central U.S." (1984) was popularized, if not coined, by Walter Mondale's presidential campaign.

1530s, from French rusticite (15c.), from Latin rusticitatem (nominative rusticitas) "country life," from rusticus (see rustic (adj.)).