impenetrable

Concise Oxford English Dictionary © 2008 Oxford University Press:
impenetrable /ɪmˈpɛnɪtrəb(ə)l/

▶adjective

• 1 impossible to pass through or enter.

• 2 impossible to understand.

• 3 Physics (of matter) incapable of occupying the same space as other matter at the same time.

– derivatives
impenetrability noun,
impenetrably adverb.

– origin ME: via Fr. from L. impenetrabilis, from in- ‘not’ + penetrabilis ‘able to be pierced’, from penetrare (see penetrate).