inscrutable
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.
inscrutable
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.
Inscrutative
Difficult to understand
Most of the languages in the world are inscrutative