Arm2  —n. 1 (usu. In pl.) Weapon. 2 (in pl.) Military profession. 3 branch of the military (e.g. Infantry, cavalry). 4 (in pl.) Heraldic devices (coat of arms). —v. 1 supply, or equip oneself, with weapons etc., esp. In preparation for war. 2 make (a bomb etc.) Ready.  take up arms go to war. Under arms equipped for war. Up in arms (usu. Foll. By against, about) actively resisting, highly indignant. [latin arma arms]