The Spanish word 'arma' (meaning 'weapon') comes directly from the Latin word 'arma', which meant 'weapons' or 'tools of war'. This Latin word 'arma' is itself related to the Latin word 'armum', meaning 'arm' or 'shoulder', which makes intuitive sense since weapons are typically wielded by the arms. The connection between arms (body parts) and weapons has been preserved in many languages descended from Latin.

Some related Spanish words include 'armado' (armed), 'armadura' (armor), and 'desarmar' (to disarm). All these words share the basic root 'arma' and relate to weapons or military equipment.

English speakers might recognize the connection to words like 'arm' (as in weapons), 'armory', 'armor', and 'army'. While in English we use 'arm' to mean both the body part and a weapon, these words all trace back to the same Latin roots as Spanish 'arma'. The military meaning has remained remarkably consistent across thousands of years and multiple languages.