Sacred cow is an idiom, a figurative reference to sacred cows in some religions. This idiom is thought to originate in American English, although similar or even identical idioms occur in many other languages.
The idiom is based on the popular understanding of the elevated place of cows in Hinduism and appears to have emerged in America in the late 19th century. A literal sacred cow or sacred bull is an actual cow or bull that is treated with sincere respect. A figurative sacred cow is a figure of speech for something considered immune from question or criticism, especially unreasonably so.
Paradox
There is an element of paradox in the concept of respect for a sacred cow, as illustrated in a comment about the novelist V. S. Naipaul: V. S. Naipaul ... has the ability to distinguish the death of an ordinary ox, which, being of concern to no one, may be put quickly out of its agony, from that of a sacred cow, which must be solicitously guarded so that it can die its agonizing death without any interference.
"Irreverence is our only sacred cow" is the motto of The Realist, an irreverent magazine.
