1. That which is impossible to pierce, to get into, or to get through: The wall was impenetrable despite all of the bullets that were fired into it.

The thorny branches made a thick, impenetrable hedge around Helena's house.

2. A reference to something that is inconceivable to understand or to explain: Lee was reading about what the police declared to be an impenetrable crime.

The students couldn't comprehend the teacher's impenetrable explanation of the mathematical processes.

3. Not open to ideas, impressions, influences, etc.: Ingrid had an impenetrable mind and so she would not change her opinions regarding how to proceed with the project.
4. Etymology: borrowed through Middle French impenetrable; from Latin impenetrabilis, from im-, "not, opposite of", a variant of in- before p + penetrabilis, "put into, get into, enter into".