What is the origin of the idiom "holy cow" and when to use it?

The idiom "holy cow" is used to express surprise, confusion, or amazement. It is typically used in informal speech and is often used to add emphasis to a reaction or to convey a sense of wonder. The origin of this expression is not precisely documented, but it has been in use for many years and is thought to have evolved as a lighthearted way of expressing surprise without resorting to strong language.