the mind > mental capacity > knowledge > want of knowledge, ignorance > unfamiliarity with, inexperience > lack skill or experience [verb] (5)
transitive. To lack direct experience of or familiarity with; not to be acquainted or conversant with (a person, place, or thing). Also: to lack…
Of a person, his faculties, etc.. Wanting or deficient in, †of; without skill or learning in. With for: Badly off or at a loss for, unable to find. ar…
Originally U.S. not to know the first thing about: not to know even the elements or rudiments of.
Colloquial phrase not to have a clue: to have no idea; not to know; to be ignorant or incompetent.
not to have a scooby: = not to have a clue at clue, n. 2e.