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1.
If a person or their expression is inscrutable, it is very hard to know what they are really thinking or what they mean.
In some circumstences, it is important to keep a straight face and remain inscrutable.
by Poljak October 26, 2007
 
2.
An inscrutable girl is one who'll make out, but not put out.
Mona and I got to third base, but she proved to be totally inscrutable.
by scodder June 07, 2010