'ferruginous' originates from Latin, specifically the word 'ferruginosus', where 'ferrugo' meant 'rust' or 'iron rust' and the suffix '-osus' meant 'full of'.

'ferruginosus' in Latin passed into Late Latin and then into Middle English as 'ferruginous', retaining a similar form and entering English usage with meanings related to rust and iron.

Initially, it meant 'full of rust' or 'rusty'; over time it broadened to mean both 'containing iron' (chemical/content sense) and 'rust-colored' (color sense).