Adjective impregnable has 2 senses
  1. impregnable, inviolable, secure, strong, unassailable, unattackable - able to withstand attack; "an impregnable fortress"; "fortifications that made the frontier inviolable"
    Antonym: vulnerable (indirect, via invulnerable)
  2. impregnable, inexpugnable - impossible to take by storm
    Antonym: conquerable (indirect, via unconquerable)
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