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One that is immune from criticism, often unreasonably so: "The need for widespread secrecy has become a sacred cow" (Bulletin of the Atomic Scientists).
[From the veneration of the cow by the Hindus.]
informal a person, institution, custom, etc, unreasonably held to be beyond criticism
[alluding to the Hindu belief that cattle are sacred]
someone or something considered to be exempt from criticism or questioning.
[1905–10; in reference to the traditional inviolability of the cow among Hindus]