rust (n.) Look up rust at
"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," rudeti "to rust;" Latin robigo, Old Church Slavonic ruzda "rust"), from root *reudh- "red" (see red (adj.1)).

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.
rust (v.) Look up rust at
early 13c., from rust (n.). Transitive sense "cause to rust" is from 1590s. Related: Rusted; rusting.