acero
/aˈθeɾo/ · B1 · noun · m
steel
⚔️ acero sounds like a sharp edge: Latin aciarium, from acies = blade's edge
From Late Latin aciarium, built on acies (sharp edge, point), the same root as English acid and acute. Steel is the metal that holds a sharp edge, so acero came to mean the sword too, and by extension arms in general.