The story
Originally American English (early 1900s), a reference to the elevated place of cows in Hinduism.
Etymology from Wiktionary, CC BY-SA 4.0.
sacred cow
Today it means: 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. Full definition →