Open English Wordnet
Adjectives
(s)impregnable, inviolable, secure, strong, unassailable, unattackableimmune to attack; incapable of being tampered with“an impregnable fortress”, “fortifications that made the frontier inviolable”, “a secure telephone connection”
(s)unassailable, unshakable, watertight, bulletproof, ironcladwithout flaws or loopholes“an ironclad contract”, “a watertight alibi”, “a bulletproof argument”