a.

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

   装甲板普通子弹打不穿。

   the impenetrable forest

   无法通过的森林

2. 不能理解的；难以探测的
   The motive for the crime was impenetrable.

   那起罪行的作案动机叫人捉摸不透。

3. 不接纳的；顽固的[（+to/by）]
   He is impenetrable to reason.

   他不讲道理。

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.