A ghost or supernatural being; also slang for a spy or to frighten someone suddenly.
From Dutch 'spook' or German 'Spuk,' entering English in the 19th century. The Dutch word may derive from Germanic roots meaning 'to move or appear suddenly.' The spy meaning developed in 20th-century American slang.
The word 'spook' reveals how American English borrowed from Dutch immigrants during the colonial period, and how the same word for 'ghost' evolved into espionage slang—possibly because spies move suddenly and invisibly like ghosts.
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