Meaning : Immune to attack. Incapable of being tampered with.
Example :
An impregnable fortress.
Fortifications that made the frontier inviolable.
A secure telephone connection.
Synonyms : impregnable, inviolable, secure, strong, unattackable
Meaning : Impossible to assail.
Synonyms : untouchable
Meaning : Without flaws or loopholes.
Example :
An ironclad contract.
A watertight alibi.
A bulletproof argument.
Synonyms : bulletproof, unshakable, watertight