a.

1. 不能通過的；不能穿過的；不能刺穿的；不透……的[（+to/by）]
   Armor plate is impenetrable by ordinary bullets.

   裝甲板普通子彈打不穿。

   the impenetrable forest

   無法通過的森林

2. 不能理解的；難以探測的
   The motive for the crime was impenetrable.

   那起罪行的作案動機叫人捉摸不透。

3. 不接納的；頑固的[（+to/by）]
   He is impenetrable to reason.

   跟他道理說不通。

4. 【物】不可入性的

adj.

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.