Armor-plated means having a protective layer of metal or other strong material, making something virtually indestructible. The antonyms of armor-plated could be exposed, unprotected, vulnerable, weak, or feeble. Something that is not armor-plated is more susceptible to damage or harm from outside impact. For instance, an unarmored car may not withstand a crash as well as an armored car. Similarly, a weak argument without any supporting evidence may not hold up against criticism. An exposed city may be more vulnerable to an enemy attack than a fortified one. In short, the antonyms of armor-plated connote the lack of protection and defense.