impenetrable

Etymology

From Middle French impenetrable, from Latin impenetrabilis.

adj

1. 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. 2012, John Branch, “Snow Fall : The Avalanche at Tunnel Creek”, in New York Time
2. (figurative) Incomprehensible; fathomless; inscrutable.
   Business jargon makes this document impenetrable, I can't understand it.
3. Opaque; obscure; not translucent or transparent.
   When night falls, she cloaks the world in impenetrable darkness.

noun

1. A person not openly given to friendship.