The word 'spook' entered English in the early 19th century from Dutch, where it has long signified a specter or ghost. The Dutch term itself is believed to be of a Germanic origin, possibly related to a Middle Low German word meaning "apparition" or "fright," and may be connected to a Proto-Germanic root expressing the concept of spectral haunting. Its journey into American English was likely reinforced by Dutch settlers' influence. The semantic extension to mean "spy" in the 20th century arose from the secretive, shadowy, and unsettling nature of espionage work, metaphorically linking the covert agent to an unseen, ghostlike presence.

This word is a simplex loanword. It is borrowed from Dutch, where 'spook' means "ghost" or "apparition." Its functional role is as a complete lexical unit.