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Latin word arma comes from Proto-Indo-European *h₂r̥-ti-
*h₂r̥-ti- (Proto-Indo-European)
*h₂(e)rmos (Proto-Indo-European)
fitting
arma (Latin)
(Late Latin) weapon (pluralonly) defence. (pluralonly) defensive arms, armor/armour, shields, weapons of war.. (pluralonly) soldiers, military power. (pluralonly) tools. (pluralonly) war.