ferrum

Ferrum, ferri, n., second declension. The metal, and then by metonymy anything made of it, which in practice means the sword: ferrum stringere is to draw a blade, not to smelt ore, and ferro perire is to die by the sword. Ferro ignique, "by sword and fire", is the fixed formula for total devastation and turns up in every historian.

Keep it clear of fero, ferre, "to carry", which shares four letters and no ancestry. The adjective ferreus means "of iron" and then "hard, unfeeling", which is the whole of the phrase ferrea aetas. English keeps ferrous, ferric and farrier, and the chemical symbol Fe carries the word into every periodic table on earth.