"an impregnable fortress"
"fortifications that made the frontier inviolable"
"a secure telephone connection"
"an impregnable argument"
"impregnable self-confidence"
The Schloss Adler is inaccessible and impregnable. It would require a battalion of paratroops to take it.
出自: A. MacLean
She carries within an impregnable conviction of being lovable.
出自: A. Storr