holy of holies

(noun) (figurative) something regarded as sacred or inviolable. Synonyms: holy of holies. Example: "Every politician fears to touch that holy of holies, the social security system."

(noun) (Judaism) sanctuary comprised of the innermost chamber of the Tabernacle in the temple of Solomon where the Ark of the Covenant was kept. Synonyms: holy of holies, sanctum sanctorum.