arma

The root arma comes from Latin arma — "weapons, tools of war" — and its verb armāre, "to arm, to equip." Surprisingly, it shares an ancient ancestor with armus, the Latin word for "upper arm, shoulder." Both trace back to the Proto-Indo-European root ar-, meaning "to fit together, to join." The link makes sense: your arm is the limb "jointed" to your shoulder, and weapons are the gear that "fits" a soldier for battle. From this single image of fitting and equipping, English inherited two families that look identical — the body part and the war gear — and over time they fused in the word arm itself.

That is why arm carries two meanings that feel unrelated but are cousins. As a noun it is the limb (break your arm); as a verb it means "to equip with weapons" (arm the soldiers), and as a plural noun arms means "weapons" (lay down your arms). The leap is small once you see it: the limb that wields a weapon, and the weapon itself, both come from "that which is fitted on."

From armāre Latin built a whole vocabulary of war, which English absorbed through French:

- army — an armed force; the collective body of people equipped for war.
- armament — arma + -ment — the weapons and equipment of a force, taken collectively.
- armada — a Spanish form meaning an armed fleet; the famous Spanish Armada of 1588.
- armour / armor — protective gear; not weapons that strike, but the equipment that fits over a body to shield it.
- armory / armoury — the place where arma are stored.
- armistice — arma + a form of sistere "to make stand still" = "making the weapons stand still," i.e. a ceasefire.
- disarm — dis- (reverse) + arm = to take the weapons away; figuratively, to take away someone's hostility.

The pattern is steady: wherever you see arm-, picture either the limb that does the fighting or the weapons it carries. Army gathers the armed, armament counts the weapons, armour shields the fighter, armistice freezes the guns, and disarm strips them away. One Latin idea — fitting a person out for war — branches into the whole language of conflict and defense.