Shellproof refers to something that is protected or immune to damage, harm, or penetration. Some antonyms for shellproof could include vulnerable, unprotected, exposed, susceptible, or defenseless. These words suggest a lack of security, safety, or protection, leaving something or someone open to various risks, including physical harm, emotional damage, or financial loss. Examples of vulnerable situations or conditions could be an open wound, a weak immune system, lack of insurance, a large debt, or a fragile relationship. In contrast, a shellproof item or environment could be a fortified building, a secure password, a sturdy lock, or a bulletproof vest.