impregnable /im-pregˈnə-bl/

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

impregnabilˈity noun

• horseless carriages
• horse-litter
• horse-litters
• horse mackerel
• horse mackerels
• horseman
• horsemanship
• horsemanships
• horseman's word
• horse marine
• horse marines
• horsemeat
• horsemeats
• horsemen
• horse mill
• horse-milliner
• horse mills
• horsemint
• horsemints
• horse mushroom
• horse mushrooms
• horse mussel
• horse mussels
• horse nail
• horse nails