So here's the deal: Spook comes from the Dutch word for apparition, or specter. The noun was first used in English around the turn of the nineteenth century. Over the next few decades, it developed other forms, like spooky, spookish, and of course, the verb, to spook.

Why are MI5 agents called spooks?

The title is a popular colloquialism for spies, and the series follows the work of a group of MI5 officers based at the service's Thames House headquarters, in a highly secure suite of offices known as The Grid.

What is a spook in military terms?

Spook. Slang for intelligence officer. The primary meaning of spook is a ghost. Like them, intelligence officers operate in the shadows.