Any program, policy, or person that is regarded as being beyond attack or untouchable. The term references the status held by cows in Hindu culture, where the cow is regarded as a sacred animal.
In American politics, Social Security has been considered a sacred cow because it is so politically popular that most politicians would never support ending the program.
“Sacred Cow” was also the nickname of the first military aircraft used to transport a United States president. According to the National Museum of the Air Force, President Franklin D. Roosevelt flew in the Sacred Cow to meet Prime Minister Winston Churchill, and General Secretary Joseph Stalin in the USSR for the Yalta Conference in February 1945.