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.一个古怪、难以捉摸、反复无常的人...the dark eyes that right now seemed opaque and unfathomable.此刻显得浑浊莫测的黑眼睛
英汉词典释义
adj.
深得无法到达底部的
the unfathomable depths不可能测量的深度an unfathomable chasm深渊
高深莫测的;难以了解的
英英词典释义
Adjective
1. of depth; not capable of being sounded or measured