A gremlin is a mischievous mythical creature that's said to cause trouble to machinery, particularly in airplanes. When you're a nervous flyer, the last thing you want to think about is gremlins messing with the plane's engines.
The word gremlin first appeared in print in 1929. It emerged from the U.K.'s Royal Air Force aviator slang (possibly influenced by goblin), and it became especially popular during World War II. Gremlins were described as tricksters that could be blamed for engine malfunctions and other mysterious, unexplained mechanical problems. Gremlin was also the name of a Marvel Comics character and a 1970s AMC car, and Gremlins was a popular 1984 film.