1. not admitting of penetration or passage into or through

- an impenetrable fortress

- impenetrable rain forests

Similar word(s): dense, thick

2. permitting little if any light to pass through because of denseness of matter

- impenetrable gloom

Similar word(s): thick, dense, heavy

3. impossible to understand

- impenetrable jargon

Similar word(s): incomprehensible, uncomprehensible

Sentences with impenetrable as an adjective:

- The fortress is impenetrable, so it cannot be taken.

- Business jargon makes this document impenetrable, I can't understand it.