impregnable /im-pregˈnə-bl/

1. That cannot be captured, broken into or taken by force
2. Which cannot be overcome, unassailable

impregnabilˈity noun

• surplus
• surplusage
• surplusages
• surpluses
• surprisal
• surprisals
• surprise
• surprised
• surprisedly
• surpriser
• surprisers
• surprises
• surprise, surprise
• surprising
• subreferences
• subregion
• subregional
• subregions
• subreption
• subreptions
• subreptitious
• subreptive
• subring
• subrings
• subrogate