A will-o’-the-wisp (/ˌwɪl ə ðə ˈwɪsp/), will-o’-wisp (/ˌwɪl ə ˈwɪsp/), or ignis fatuus (/ˌɪɡnɪs ˈfætʃuəs/); Medieval Latin for “foolish fire”)
1a. thing that is impossible to obtain; a person that you cannot depend on;
2a. blue light that is sometimes seen at night on soft wet ground and is caused by natural gases burning.
– WORD ORIGIN
Early 17th cent.: originally as Will with the wisp, the sense of wisp being ‘handful of (lighted) hay’.
(Taken from “Oxford Advanced Learner’s Dictionary 9th edition © Oxford University Press, 2015”)
