Ignis Fatuus

A wavering luminous appearance frequently observed in meadows and marshy places, around which many popular superstitions cluster. Its folknames, Will o' the Wisp and Jack o' Lantern, suggest a country fellow bearing a lantern or straw torch (wisp). Formerly these lights were supposed to haunt desolate bogs and moorlands for the purpose of misleading travelers and drawing them to their death. Another superstition says that they are the spirits of those who have been drowned in the bogs, and yet another says that they are the souls of unbaptized infants. Science now attributes these ignes fatui to gaseous exhalations from the moist ground or, more rarely, to night-flying insects.

ignis fatuus a will-o'-the-wisp. Recorded in English from the mid 16th century, the phrase is modern Latin, literally ‘foolish fire’ (because of its erratic movement).

ignis fatuus will-o'-the-wisp. XVI. modL, ‘foolish fire’, so named from its erratic flitting from place to place.