If you describe a belief, custom, or institution as a sacred cow, you disapprove of people treating it with too much respect and being afraid to criticize or question it.
[disapproval]
...the sacred cow of monetarism. [+ of]
sacred cow in British English
noun
informal
a person, institution, custom, etc, unreasonably held to be beyond criticism
Word origin
alluding to the Hindu belief that cattle are sacred
sacred cow in American English
US
any person or thing regarded as above criticism or attack