sacred cow

If a person is so highly respected that it feels wrong to criticize him, you can call him a sacred cow.

When you're hesitant to raise objections to something — a badly planned but popular school fundraiser, for example — you can call the project a sacred cow. The phrase comes from the belief of devout Hindus that cows are sacred animals and should never be harmed. The earliest American usage of sacred cow to mean "immune from criticism" was in the late 1800s.

Definitions of sacred cow
1

n a person unreasonably held to be immune to criticism

Type of:
important person, influential person, personage
a person whose actions and opinions strongly influence the course of events

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