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1. A ghost or spirit, especially one that is or seems malicious. Primarily heard in US. My grandfather says this old house is filled with all manner of spooks, but I don't believe in that sort of thing.
2. slang An undercover intelligence agent; a spy. Primarily heard in US. I worked as a spook in that country for nearly 40 years, gathering intel on every aspect of the government's operations. Everyone knows there are spooks in our country, just like we have spooks countries all over the world.
3. offensive slang A derogatory term for a black person. Primarily heard in US.
4. verb To startle, unnerve, or unsettle someone or something. Sorry, I didn't mean to spook you! Something in these woods has spooked the horses. I can barely keep them under control! The company's downward revision of their expected sales seems to have spooked investors, as stock prices dropped by nearly 4 points this morning.
mod. frightened or startled. The guy looked sort of spooked. He was sweating and panting like someone had scared him to death.
See also: spook