An idle mind loses its tone and strength like an unused muscle; the mental powers go to rust through idleness and inaction.
"red oxide of iron," Old English rust "rust; moral canker," related to rudu "redness," from Proto-Germanic *rusta- (cf. Frisian rust, Old High German and German rost, Middle Dutch ro(e)st), from PIE *reudh-s-to- (cf. 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.
early 13c., from rust (n.). Transitive sense "cause to rust" is from 1590s. Related: Rusted; rusting.
Any of a group of parasitic fungi of the order Uredinales that are plant pathogens, especially of cereal grains, and that can produce allergy in humans when inhaled in large numbers.
Verb To become corroded or oxidized.