"impregnable" in the adjective sense
1. 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"
2. conceptive, impregnable
capable of conceiving
3. impregnable, inexpugnable
incapable of being overcome, challenged or refuted
"an impregnable argument"
"impregnable self-confidence"
Source: WordNet® (An amazing lexical database of English)
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