Plural of spook; ghosts or spirits, or informally, intelligence agents and spies.
From Dutch 'spook' or German 'Spuk,' both meaning ghost or apparition. The term entered English in the 19th century. The slang meaning for spies developed later, possibly from the secretive, ghostlike nature of spy work.
The CIA and other intelligence agencies have been jokingly called 'spooks' for so long that the word actually appears in official government documents—it started as slang for people who work in shadows, just like ghosts do!
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