'armor-plated' is a Modern English compound formed from the noun 'armor' and the past-participial adjective-forming element 'plated' (from 'plate'), both ultimately borrowed via Old French and Late Latin roots.

'armor' came into English via Middle English 'armour' from Old French 'armure' (meaning equipment for war/protection); 'plate' comes from Old French 'plate' (flat metal), from Late Latin 'plattus' (flat). The compound 'armor-plated' developed in Modern English as a descriptive adjective.

Originally the separate elements meant 'protective equipment' ('armor') and 'a flat metal piece' ('plate'); combined in Modern English they came to mean 'fitted with protective metal plating' (i.e., covered by metal armor).