British Dictionary definitions for will o' the wisps'
Also called friar's lantern, ignis fatuus, jack-o'-lantern. a pale flame or phosphorescence sometimes seen over marshy ground at night. It is believed to be due to the spontaneous combustion of methane or other hydrocarbons originating from decomposing organic matter
a person or thing that is elusive or allures and misleads
will-o'-the-wispish, will-o'-the-wispy, adjective
C17: originally Will with the wisp, from Will short for William and wisp in former sense of a twist of hay or straw burning as a torch