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Phrase(s): sacred cow
Fig. something that is regarded by some people with such respect and veneration that they do not like it being criticized by anyone in any way. (From the fact that the cow is regarded as sacred in India and is not eaten or mistreated.) • A university education is a sacred cow in the Smith family. Fred is regarded as a failure because he quit school at 16. • Don’t talk about eating meat to Pam. Vegetarianism is one of her sacred cows.
McGrawhill's American Idioms And Phrasal Verbs
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A person or thing immune to criticism or questioning, as in The rules governing the press conference have become a sacred cow in this administration. This term alludes to the honored status of cows in Hinduism, where they are a symbol of God's generosity to humankind. It has been used figuratively since about 1900.
American Heritage Idioms