Developing rust (a reddish-brown coating of iron oxide) when iron or steel is exposed to air and moisture.
From the noun 'rust' (Old English 'rust'), possibly from Proto-Germanic roots meaning 'red.' The '-ing' suffix makes the present participle. Rust as oxidation has been observed since humans worked with metal.
Rusting is actually a form of combustion—iron slowly burns when exposed to oxygen, just like fire does, but at room temperature and invisibly slow! This is why rust spreads so aggressively once it starts.
Complete word intelligence in one call. Free tier — 50 lookups/day.