9 Grammatical Pitfalls
Figuratively, anything that is beyond criticism: “That housing project is a real sacred cow: the city council won't hear of abandoning it.” In India, followers of Hinduism consider cows sacred and do not eat them because they believe the animals contain the souls of dead persons.
Someone or something that may not be questioned or altered: Everything is on the table. There are no sacred cows
[1910+; fr the venerated status of cows in Hinduism]