will-o'-the-wisp

 
Pronunciation: /wɪləðəˈwɪsp/

noun

  • a phosphorescent light seen hovering or floating at night on marshy ground, thought to result from the combustion of natural gases.
  • a person or thing that is difficult or impossible to reach or catch.

Origin:

early 17th century: originally as Will with the wisp, the sense of wisp being 'handful of (lighted) hay'