rūst

Old English rūst comes from Proto-Germanic *rustą, from Proto-Indo-European *h₁rewdʰ- (red). The same root produced Latin ruber (red), Russian рыжий (ryzhiy, red-haired), and English 'red' itself. Rust was named for what it looked like: the red stuff on iron. The chemical process — iron reacting with oxygen and water to form iron oxide — was not understood until modern chemistry. For thousands of years, rust was simply the redness that ate metal.

Rust was iron's curse. The metal that built the Iron Age was also the metal that corroded most aggressively. Bronze does not rust (it develops a stable green patina). Copper does not rust. Gold does not rust. Only iron and its alloys rust in the destructive sense — forming flaky, crumbling oxide that weakens the metal progressively. The word 'rust' became a synonym for decay because iron's decay was the most visible and most costly.

The Rust Belt — the deindustrialized region of the northeastern and midwestern United States — was named in the 1980s for the abandoned, rusting factories that marked the landscape. The word transferred from chemistry to geography to politics in a single metaphor. A region was named for what its factories looked like when the factories closed. The economic decay was made visible by the chemical decay of the iron structures that remained.

The programming language Rust (2010, developed by Mozilla) was named partly as a reference to the rust fungus — a biological organism, not chemical oxidation — and partly as a play on being a 'bare metal' language (close to the hardware). The word has entered technology from metallurgy, and the programmers who chose it knew exactly what they were saying: Rust is a systems language. It operates where metal meets code.