a mischievous invisible being, said by airplane pilots in World War II to cause engine trouble and mechanical difficulties.
2.
any cause of trouble, difficulties, etc.
Origin: 1925–30; of obscure orig; in its earliest attested use, an RAF term for a low-ranking officer or enlisted man assigned the most onerous duties; later development perh. affected by phonetic resemblance to goblin
An imaginary gnomelike creature to whom mechanical problems, especially in aircraft, are attributed.
A maker of mischief.
[Perhaps blend of Irish gruaimín, bad-tempered little fellow (from Middle Irish gruaim, gloom, surliness) and goblin.]
Word History: Elves, goblins, and trolls seem to be timeless creations of the distant past, but gremlins were born in the 20th century. In fact, gremlin is first recorded only in the 1920s, as a Royal Air Force term for a low-ranking officer or enlisted man saddled with oppressive assignments. Said to have been invented by members of the Royal Naval Air Service in World War I, gremlin is used in works written in the 1940s for "an imaginary gnomelike creature who causes difficulties in aircraft." The word seems likely to have been influenced by goblin, but accounts of its origin are various and none are certain. One source calls in Fremlin beer bottles to explain the word; another, the Irish Gaelic word gruaimín, "ill-humored little fellow." Whatever the word's origin, it is certain that gremlins have taken on a life of their own.
[grem-lin] 
lɪn