impregnable, inviolable, secure, strong, unassailable, unattackable -- (immune to attack; incapable of being tampered with; "an impregnable fortress"; "fortifications that made the frontier inviolable"; "a secure telephone connection")
conceptive, impregnable -- (capable of conceiving)
impregnable, inexpugnable -- (incapable of being overcome, challenged or refuted; "an impregnable argument"; "impregnable self-confidence")
(Sound)
