Answer: impenetrable

AnswerCrossword Clue
IMPENETRABLEQuite secure
impenetrable(of matter) Incapable of occupying the same space as other matter at the same time
impenetrableImpossible to understand
impenetrableImpervious to new ideas or influences
impenetrableImpossible to pass through or enter
impenetrable(of a club or group) Secretive and exclusive
impenetrableincapable of being penetrated or pierced