Inscrutable

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The American Heritage^® Dictionary of the English Language: Inscrutable
Difficult to fathom or understand; impenetrable
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Oxford Advanced Learner’s Dictionary:
if a person or their expression is inscrutable, it is hard to know what they are thinking or feeling, because they do not show any emotion
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MB:

Mona Lisa's Inscrutable Smile
The painting was analysed by a University of Amsterdam computer using "emotion recognition" software.
It concluded that the subject was 83% happy, 9% disgusted, 6% fearful and 2% angry, New Scientist magazine was told. BBC News Article: Mona Lisa 'happy', computer finds

The usage of this word -

• Inscrutable Americans.
• FUTURE IS INSCRUTABLE.
  

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Google News: Inscrutable