From Old English 'rūst,' from PIE *reudh- (red) — rust is etymologically 'the red stuff,' named purely for its color.
A reddish-brown coating formed on iron by oxidation, especially in the presence of moisture.
From Old English 'rūst' meaning 'rust,' from Proto-Germanic *rustą, from PIE *reudh- meaning 'red.' Rust is etymologically the 'red stuff' — named for its color. Key roots: *reudh- (PIE: "red").