‖Ig″nis fat″u‐us (?); pl.Ignes fatui (#). [L. ignis fire + fatuus foolish. So called in allusion to its tendency to mislead travelers.] 1. A phosphorescent light that appears, in the night, over marshy ground, supposed to be occasioned by the decomposition of animal or vegetable substances, or by some inflammable gas; — popularly called also Will-with-the-wisp, or Will-o'-the-wisp, and Jack-with-a-lantern, or Jack-o'-lantern.
2. Fig.: A misleading influence; a decoy.
Scared and guided by the ignis fatuus of popular superstition. Jer. Taylor.