will-o’-the-wisp
noun/ˌwɪl ə ðə ˈwɪsp/
/ˌwɪl ə ðə ˈwɪsp/
[usually singular]- a thing that is impossible to obtain; a person that you cannot depend on
难以捉摸的人（或事物）；镜花水月；虚无之物 - a blue light that is sometimes seen at night on soft, wet ground and is caused by natural gases burning
磷火；鬼火
Word Originearly 17th cent.: originally as Will with the wisp, the sense of wisp being ‘handful of (lighted) hay’.