sacred cow

noun1

1. •
   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.

   • Many sacred cows were milked dry in an unbelievable series of outrageous events. — 1965