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)impregnable, inexpugnableincapable of being overcome, challenged or refuted“an impregnable argument”, “impregnable self-confidence”