impenetrable

Not penetrable.

The fortress is impenetrable, so it cannot be taken.

The avalanche spread and stopped, locking everything it carried into an icy cocoon. It was now a jagged, virtually impenetrable pile of ice, longer than a football field and nearly as wide.

Opaque; obscure; not translucent or transparent.

When night falls, she cloaks the world in impenetrable darkness.

Incomprehensible; fathomless; inscrutable.

Business jargon makes this document impenetrable—I can’t understand it.

A person not openly given to friendship.

I should lose the reputation that I am gradually acquiring among our impenetrables here, were I to confess the excitement which I felt at the idea of entering his house—the house of that great general under whose command you made your first charge.