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?yàn)槟撤N不可理解的原因，該有樓梯的地方卻沒(méi)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(cè)的[文學(xué)性]

...a strange, unfathomable and unpredictable individual. ...一個(gè)奇怪的、高深莫測(cè)且難以預(yù)料的人。