'iron-poor' is a Modern English compound formed from the noun 'iron' and the adjective 'poor'. 'iron' ultimately comes from Old English 'īsarn' (also 'īsern'), from Proto-Germanic *īsarną; 'poor' comes from Old French 'povre' (from Latin 'pauper').

'iron' developed from Old English 'īsarn' → Middle English 'iren' → modern English 'iron'. 'poor' came from Old French 'povre' → Middle English 'poure/poor'. The compound 'iron-poor' arose in Modern English by combining the two words to describe a lack of iron.

Originally the components referred separately to the metal 'iron' and the state 'poor'; as a compound, 'iron-poor' came to mean specifically 'deficient in iron', a usage popularized in medical, nutritional, and agricultural contexts.