/ɪmˈpen.ə.trə.bəl/
If you describe something such as a book or a theory as impenetrable, you are emphasizing that it is impossible or very difficult to understand.
an impenetrable mystery
不解之谜
~ to sb
Their jargon is impenetrable to an outsider.
他们的行话外人听不懂。
that cannot be entered, passed through or seen through 不可进入的；穿不过的；无法透视的
an impenetrable jungle
无法穿越的丛林
impenetrable darkness
漆黑