unknown:

adj. 未知的；陌生的，默默无闻的

n. 未知数；未知的事物，默默无闻的人

His work is practically unknown here .
他的作品在这里实际上不为人知。

What will the unknown devil bring ?
这个摸不透的恶魔会带来什么呢？

The date of birth and death of qu yuan is unknown .
屈原的生卒年月不详。

His whereabouts have been utterly unknown so far .
迄今他的下落全然未知。

His present whereabouts is unknown .
他现在下落不明。

unknown to sb.

背着某人；在某人不知晓的情况下,

例句:

Unknown to her, I had already seen my lawyer.(我已经背着她见过我的律师了。)