Definitions for sacred cow
Random House Webster's College Dictionary
someone or something considered to be exempt from criticism or questioning.
Origin of sacred cow:
1905–10; in reference to the traditional inviolability of the cow among Hindus
a person unreasonably held to be immune to criticism
Something which cannot be tampered with, or criticized, for fear of public outcry. A person, institution, belief system, etc. which, for no reason other than the demands of established social etiquette or popular opinion, should be accorded respect or reverence, and not touched, handled or examined too closely.