impregnable - definition and synonyms

adjective 

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/ɪmˈpreɡnəb(ə)l/
  1. 1
    a place that is impregnable is very well protected and difficult to attack
  2. 2
    someone who is in an impregnable position is very difficult to defeat or remove

    The Prime Minister’s position is no longer impregnable.

  3. 3
    impregnable opinions or feelings are very difficult to change