untouchable

beyond the reach of criticism or attack or impeachment; "for the first time criticism was directed at a hitherto untouchable target"- Newsweek

impossible to assail

forbidden to the touch; "in most museums such articles are untouchable"

( especially used in traditional Hindu belief of the lowest caste or castes) defiling

not capable of being obtained; "a rare work, today almost inaccessible"; "timber is virtually unobtainable in the islands"; "untouchable resources buried deep within the earth"

belongs to lowest social and ritual class in India