impregnable
adjectiveUK /ɪmˈpreɡ.nə.bəl/
US /ɪmˈpreɡ.nə.bəl/
So strong that nobody can break in or take it over.
The castle sat on a high hill and was completely impregnable.
OTHER MEANINGS
2. Too strong to be beaten or argued against.
Her lead in the race was impregnable.
3. Able to resist any attack or damage.
The team built an impregnable defense.
4. Firm and impossible to change or shake.
His belief in fairness was impregnable.