Definition of will-o’-the-wisp noun from the Oxford Advanced Learner's Dictionary

     

    will-o’-the-wisp

     noun
    noun
    BrE BrE//ˌwɪl ə ðə ˈwɪsp//
     
    ; NAmE NAmE//ˌwɪl ə ðə ˈwɪsp//
     
    [usually singular]
     
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  1. 1a thing that is impossible to obtain; a person that you cannot depend on
  2. 2a blue light that is sometimes seen at night on soft wet ground and is caused by natural gases burning
  3. Word Origin early 17th cent.: originally as Will with the wisp, the sense of wisp being ‘handful of (lighted) hay’.
See the Oxford Advanced American Dictionary entry: will-o’-the-wisp