A sacred cow is something that is considered immune to criticism by virtue of its high status. This term is often used in a sardonic way to describe something that is regarded so highly that many people are afraid to criticize it. When discussing a sacred cow, the implication is that it must be handled very carefully to avoid causing offense. This slang term is used in many English-speaking countries, although some people find it offensive because of its origins.
As a slang term, "sacred cow" is derived from the status held by cows in Hindu culture, where the cow is regarded as a sacred animal. In India, where the Hindu faith is widespread, cows are allowed to freely wander the streets, and they must be treated with respect in accordance with their status. During the 1800s, many European visitors to India wrote about the sacred cow, and many people came to believe that cows were sacred in all Asian religions, not just Hinduism.
By the 1890s, Western newspapers were comparing people and situations to sacred cows, and by about 1910, they had gone a step further and dropped the comparison, simply referring to certain topics as sacred cows. By this time, the term had entered the English lexicon, and it became in widespread use. It is often used in political polemic, usually in a way which suggests that a person or situation could actually stand closer examination and perhaps some critical thought.
For people of the Hindu faith, this slang term could be considered offensive, because it references their religion in a rather uncomplimentary way. The term essentially makes fun of the concept of the sacred cow in Hindu faith, and it propagates the notion that cows generally are viewed as sacred throughout Asia, which is not the case. Although cows do play a role in some Asian religions, it is only in Hinduism that they are considered holy.