The word spook first appeared as an Americanism circa 1800. Borrowed directly from the Dutch spook, a descendant of the Middle Dutch spooc that was itself a close relative of the Middle Low German spok, the English noun was at first used to mean merely “ghost” or “a visible disembodied spirit.” By the end of the century, however, it had also come to mean “any frightening and seemingly preternatural creature” and was starting to take on its now lesser-known figurative sense of “a haunting or disturbing idea or prospect.” (Today, words such as specter and phantom have all but supplanted spook in denoting the aforesaid figurative meaning.) It wasn't until the early 1940s that spook acquired the additional sense of “an undercover agent or spy,” and the same decade saw the unfortunate development of the noun's offensively disparaging (and now highly indecorous) use as a term for a black person. By the way, spook also has two verb senses: the first, “to haunt, frighten, or otherwise behave like a ghost,” appeared in the English lexicon circa 1865; and the second, “to become suddenly frightened or nervous,” came into general use around 1935.
©2016 Michael R. Gates