Definition of will-o'-the-wisp noun from the Oxford Advanced American Dictionary

     

    will-o'-the-wisp

     noun
    noun
    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
See the Oxford Advanced Learner's Dictionary entry: will-o'-the-wisp