From the classic Mike Hammer film noir Kiss Me Deadly, it refers to a nebulous but potentially devastating threat that consumes a private detective or an entire nation. Much of its terror comes by way of implication. Examples include the communists of the 1950s, the One Ring and Weapons of Mass Destruction.
"Why is there no apostrophe in the great whatsit," the student asked.
"Don't ask stupid questions," the professor replied. "Its existence is simply a fact."