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.
例句
...a strange, unfathomable and unpredictable individual.一個古怪、難以捉摸、反復無常的人...the dark eyes that right now seemed opaque and unfathomable.此刻顯得渾濁莫測的黑眼睛
英漢詞典釋義
adj.
深得無法到達底部的
the unfathomable depths不可能測量的深度an unfathomable chasm深淵
高深莫測的;難以了解的
英英詞典釋義
Adjective
1. of depth; not capable of being sounded or measured