incapable of being penetrated or pierced
not open to intellectual influences or ideas
incapable of being comprehended
said of matter: unable to occupy the same space as other matter at the same time
impenetrability /-ʹbiliti/ noun
impenetrableness noun
impenetrably adv
[Middle English impenetrabel via French from Latin impenetrabilis, from in-1 + penetrabilis penetrable, from penetrare to penetrate]