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1801, from Dutch spook, from Middle Dutch spooc "spook, ghost," from a common Germanic source (cf. German Spuk "ghost, apparition," Middle Low German spok "spook," Swedish spok "scarecrow," Norwegian spjok "ghost, specter," Danish spøg "joke"), of unknown origin. Possible outside connections include Lettish spigana "dragon, witch," spiganis "will o' the wisp," Lithuanian spingu, spingeti "to shine," Old Prussian spanksti "spark."
Meaning "undercover agent" is attested from 1942. The derogatory racial sense of "black person" is attested from 1940s, perhaps from notion of dark skin being difficult to see at night. Black pilots trained at Tuskegee Institute during World War II called themselves the Spookwaffe.
1867, "to walk or act like a ghost," from spook (n.). Meaning "to unnerve" is from 1935. Related: Spooked; spooking.
To gain access electronically to a computer deceptively and perhaps illegally: I thought someone might be electronically impersonating him, a practice that is known online as ''spoofing''/ The technique is called ''spoofing'' because it fools a computer into thinking that another, friendly computer is requesting access (1990s+ Computer)