1、im- "not, opposite of" + per- "thoroughly" + cept- + -ibly(复合后缀：-ible, -ly).
2、literally "take entirely".
3、figuratively "grasp with the mind, learn, comprehend, recognize, notice".
4、如今，在英语里只使用其比喻义、引申义。

1. New problems arose imperceptibly.

不知不觉地出现了一些新问题.

来自《简明英汉词典》

2. Imperceptibly, the boat had slowly turned.

船慢慢地在转弯, 简直使人不觉得.

来自汉英文学 - 家（1-26） - 家（1-26）

3. Time fled, and she was surprised to find the year passed imperceptibly.

光阴飞逝, 她诧异地发觉一年不知不觉就过去了.

来自辞典例句

4. Miss Ley smilled almost imperceptibly.

莱伊小姐几乎察觉不出地微微一笑.

来自辞典例句

5. He approached imperceptibly the gloomy recess.

他不知不觉地走到了幽暗的山岔.

来自辞典例句