This word is an adjective. “Inscrutable” describes a situation where you’re unable to scrutinize, investigate or analyse that situation. If this word is used to describe a person, it means that it’s hard – or even impossible – to understand that person’s reaction.
“Inscrutable” is normally used with the word “smile” and “expression“:
- I don’t know if she feels anything, and her inscrutable smile doesn’t tell me anything either.
- After listening to all our comments, our manager showed an inscrutable gesture.
- The inscrutable depth of the ocean poses a danger to inexperienced swimmers.
I believe “inscrutable” comes from the word “scrutinize“. You might want to look up its origin.