Sacred cow

A sacred cow is a person, institution, idea (often a theory - then: "pet theory") or ideology that is immune (usually unreasonably so) from criticism or opposition.

Dating to 1910, the term has its origins in Indian custom. To most Hindus in India, cattle are sacred. Those with this belief will not eat beef even at risk of death through starvation. It is illegal in that country to kill a member of the genus Bos.