c. 1300, "mail, defensive covering worn in combat," also, generally, "means of protection," from Old French armeure "weapons, armor" (12c.), from Latin armatura "arms, equipment," from arma "weapons" (including defensive armor), literally "tools, implements (of war)," see arm (n.2). Figurative use from mid-14c.
Meaning "military equipment generally," especially siege engines, is from late 14c. The word might have died with jousting if not for 19c. transference to metal-sheathed combat machinery beginning with U.S. Civil War ironclads (it is first attested in this sense in an 1855 report from the U.S. Congressional Committee on Naval Affairs). Meaning "protective envelope of an animal" is from c. 1600.
mid-13c., "flat sheet of gold or silver," also "flat, round coin," from Old French plate "thin piece of metal" (late 12c.), from Medieval Latin plata "plate, piece of metal," perhaps via Vulgar Latin *plattus, formed on model of Greek platys "flat, broad" (from PIE root *plat- "to spread"). The cognate in Spanish (plata) and Portuguese (prata) has become the usual word for "silver," superseding argento via a shortening of *plata d'argento "plate of silver, coin."
From 14c. as "armor made of sheets of metal." Meaning "table utensils" (originally of silver or gold only) is from Middle English. Meaning "shallow dish on which food is served at table," now usually of china or earthenware, originally of metal or wood, is from mid-15c. Meaning "articles which have been covered with a plating of precious metal" is from 1540s.
In photography, "common rectangular piece of glass used to receive the picture," by 1840. The baseball sense "home base" is from 1857. Geological sense "nearly rigid part of the earth's lithosphere" is attested from 1904; plate tectonics is attested from 1967. Plate-glass for a superior kind of thick glass used for mirrors, shop-windows, etc., is recorded from 1729.