So strongly built or defended that it cannot be broken into, defeated, or destroyed by force.
From Latin 'impraegnabilis' (not to be taken), combining 'im-' (not) and 'praegnabilis' (capturable), used for fortresses and later figuratively.
Medieval castle designers obsessively studied what made fortifications 'impregnable,' but gunpowder invented in China rendered these stone monuments obsolete within centuries—technology always trumps architecture.
Complete word intelligence in one call. Free tier — 50 lookups/day.