impenetrable; not easily understood; mysterious
For some inscrutable reason which he refuses to disclose to me, my father is refusing to give his consent for our marriage.
in (not, opposite of)#scrutari (to examine, search) (Latin)
IN (not)+SCRUTINY (critical observation or examination). a thing that cannot be examined or understood is inscrutable. you cant SCRU (screw) a TABLE b'coz you can't have a penetration. may b b'coz it doesn't have a hole (a place to peek into)