U

inscrutable

adjective

incapable of being investigated, analyzed, or scrutinized; impenetrable.
not easily understood; mysterious; unfathomable: an inscrutable smile.
incapable of being seen through physically; physically impenetrable: the inscrutable depths of the ocean.

adjective

incomprehensible; mysterious or enigmatic

adj.

c.1500, from Late Latin inscrutabilis, from in- “not, opposite of” (see in- (1)) + scrutabilis, from scrutari “examine, ransack” (see scrutiny). Related: Inscrutably.