IPA: /ɪnˈskruːtəblnəs/
KK: /ɪnˈskrutəblnəs/
The quality of being difficult to understand or interpret; something that is mysterious or not easily figured out.
The inscrutableness of her expression made it hard to know what she was thinking.
Inscrutableness is formed from "in-" (meaning not), "scrutari" (meaning to search or examine), and "-able" (meaning capable of). The word describes something that is not capable of being searched or examined, hence, mysterious or difficult to understand.
Think of something that is 'not' ('in-') able to be 'searched' ('scrutari') — that's why inscrutableness refers to something that is mysterious or hard to understand.