will-o'-the-wisp
/ˈwɪləðəˈwɪsp/noun
1- a phosphorescent light seen hovering or floating at night on marshy ground, thought to result from the combustion of natural gases磷火, 鬼火。
1.1
- figurative a person or thing that is difficult or impossible to find, reach, or catch〈喻〉行踪不定的人; 出没无常的人; 捉摸不定的东西。
词源
early 17th cent.: originally as Will with the wisp, the sense of wisp being 'handful of (lighted) hay'.