means strictly a fatuous fire it is also called “Jack o' Lantern,” “Spunkie,” “Walking Fire,” “Will o' the Wisp,” and “Fair Maid of Ireland”. Milton calls it Friar's Lanthern, and Sir Walter Scott Friar Rush with a lantern. Morally speaking, a Utopian scheme, no more reducible to practice than the meteor so called can be turned to any useful end. (Plural, Ignes fatui.) (See Friars Lanthorn.)
“When thou annest up Gadshill in the night to catch my horse, if I
did not think thou hadst been an ignis fatuas or a ball of wildfire,
there's no purchase in money.” —
According to a Russian superstition, these wandering fires are the spirits of still-born children which flit between heaven and the Inferno.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894