unfathomable

1. ADJ If you describe something as unfathomable, you mean that it cannot be understood or explained, usually because it is very strange or complicated. (事物)不可理解的

For some unfathomable reason, there are no stairs where there should be. 因爲某種不可理解的原因，該有樓梯的地方卻沒有。

2. ADJ If you use unfathomable to describe a person or the expression on their face, you mean that you cannot tell what they are thinking or what they intend to do. (人、表情)莫測的[文學性]

...a strange, unfathomable and unpredictable individual. ...一個奇怪的、高深莫測且難以預料的人。