The term burden-proof means something that is so solid that it can withstand any form of criticism or argument. Some synonyms for burden-proof include impervious, bulletproof, indestructible, invulnerable, and impregnable. These terms refer to the strength of an argument or evidence that cannot be disputed or contradicted. For example, an argument that is impervious is one that cannot be penetrated by criticism or evidence against it. Bulletproof suggests that an argument is so strong that it can withstand any attack and remain valid. Indestructible refers to something that cannot be destroyed or broken regardless of the outside forces. Invulnerable and impregnable mean that something is immune to harm or attack.