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.
<-- thought to be caused by phosphine, PH3, a sponaneously combustible gas. -->
2.
Fig.: A misleading influence; a decoy.
Scared and guided by the ignis fatuus of popular superstition.
Jer. Taylor.
© Webster 1913.