c.1300, "mail, defensive covering worn in combat," also "means of protection," from Old French armeure "weapons, armor" (12c.), from Latin armatura "arms, equipment," from arma "arms, gear" (see arm (n.2)). Figurative use from mid-14c.
Meaning "military equipment generally," especially siege engines, is late 14c. The word might have died with jousting if not for late 19c. transference to metal-shielded machinery beginning with U.S. Civil War ironclads (first attested in this sense in an 1855 report from the U.S. Congressional Committee on Naval Affairs).