Gremlins were a type of fairy, or "little folk" in European legends. Although seldom seen, gremlins were known to be mischievous. If something in your home was broken or out of place with no explanation, the culprit was obviously a gremlin. This idea took on a whole new meaning when airplanes were developed, particularly in the military. When something went inexplicably haywire on a plane during World War I, the Royal Air Force blamed it on sabotage by gremlins. The idea spread to more countries by World War II, when planes were more numerous, more complicated, and under battle conditions were more likely to malfunction.
The civilian world took up the idea, too, and gremlins were used as shorthand for any kind of industrial accident or malfunction. It may have been used in early computer technology a bit, but those folks settled on blaming "bugs" instead. Gremlins went on to star in a book by Roald Dahl and a hit 1984 movie. Read about the rise of gremlins at Atlas Obscura.
(Image credit: National Archives)